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Papers by Jan Allen

ChemElectroChem

The synthesis and electrochemical insertion of lithium into the Wadsley–Roth NaNb13O33 phase is s... more The synthesis and electrochemical insertion of lithium into the Wadsley–Roth NaNb13O33 phase is studied. Lithium intercalation to form LixNaNb13O33 reaches a value of up to x~15, between 3.0 and 1.0 V vs. Li+/Li at a slow cycling rate, a capacity of 233 mAh g−1. Within this voltage window, two sharp peaks and one broad peak are observed in the differential capacity plots of lithium intercalation suggesting multiple two‐phase regions. High Li‐ion conductivity and rate capability was demonstrated. The lithium diffusion constant is about an order of magnitude greater than TiNb2O7. The average voltage is about 1.6 V and its high‐rate capability makes NaNb13O33 potentially useful as an anode in a fast‐charge Li‐ion battery application.

ECS Meeting Abstracts

Unlike batteries in classical experimental settings, batteries in practical applications are gene... more Unlike batteries in classical experimental settings, batteries in practical applications are generally not discharged immediately upon reaching a fully charged state but remain for a period of time before usage. Such a state is demanding on the cathode as much of the Ni-rich layered oxide degradation mechanisms occur at the deeply charged state. Here, we examine a Ni-rich cathode material (i.e., Li[Ni0.90Co0.05Mn0.05]O2) under conditions simulating real-use behavior of batteries and find that the addition of a short dwell period at the deeply charged state leads to substantial differences in cycling performance (89.4 % versus 37.5 % retention rates after 100 cycles, respectively). Furthermore, to overcome the rapid deterioration, Sb was used as a dopant to promote stability, especially at the grain boundaries regions, to suppress degradation at the cathode-electrolyte interface. The resulting cathode energy material (i.e., Li[Ni0.895Co0.05Mn0.05Sb0.05]O2) proved to be stable during ...

ECS Meeting Abstracts, 2016

The energy storage capability of a battery scales with the potential difference between its elect... more The energy storage capability of a battery scales with the potential difference between its electrodes. Yet operation of positive electrode materials at high potentials introduces challenges of stabilization of charged states. As of today, no positive electrode material has been demonstrated to durably and safely operate above 4.5 V vs. Li+/Li0. LiCoPO4-based electrodes theoretically offer high specific capacity and high potentials of operation, around 4.8V vs. Li+/Li0, but these electrodes are prone to failure during cycling. Failure occurs through chemical and structural degradation in the bulk of the active material or at its interfaces with cell components, especially the electrolyte. The development of Li-ion battery electrodes operating at high potential is indispensable to meet the specific energy target of 250 kWh/kg at the packaged cell level. Changes in the electronic structure and chemical stability of olivine-type LiCoPO4 and Fe-substituted LiCoPO4 were explored as both ...

ECS Meeting Abstracts

Solid Li-ion conducting electrolytes are one pathway towards future, energy-dense, intrinsically-... more Solid Li-ion conducting electrolytes are one pathway towards future, energy-dense, intrinsically-safe solid-state batteries. Thus, it is of interest to study novel fast Li-ion solid electrolyte materials. Here we report the synthesis and characterization of a family of spinel structured oxide solid electrolytes[1]. Further, we will report on the interface and mixed electronic-ionic conductivity of the spinel structured solid solution which is formed upon reaction with spinel structured Li-ion battery electrode materials. We detail the compositions that were explored and give the results of the synthesis, densification and the structural, physical and electrochemical characterization. The properties of the new materials will be compared and contrasted to well-known solid Li electrolytes such as garnet and NASICON structured materials. References 1. Allen, J.L.; Crear, B.A.; Choudhury, R.; Wang, M.J.; Tran, D.T.; Ma, L.; Piccoli, P.M.; Sakamoto, J.; Wolfenstine, J. Fast Li-Ion Conduct...

Electrochemical and Solid-State Letters, 2003

The electrochemical intercalation of unsolvated lithium cations into polyparaphenylene (PPP) has ... more The electrochemical intercalation of unsolvated lithium cations into polyparaphenylene (PPP) has been carried out in a EC/PC (1/1 volume) mixture using LiClO 4 as lithium salt. It leads to formation of saturated materials with composition Li 0.5 (C 6 H 4). Cyclic voltammetry and galvanostatic measurements reveal three successive phase transformations in the intercalation-deintercalation process. The irreversible side reactions which occur during the first PPP reduction, like decomposition of the electrolytes (EC and PC) and reduction of the binder (PVDF), are responsible for an important capacity loss that decreases rapidly in the following cycles. The cells exhibit finally a good reversibility.

ACS Applied Materials & Interfaces, 2020

In an effort to improve the cycle life and rate capability of olivine LiCoPO 4 , Cr, Fe, and Si w... more In an effort to improve the cycle life and rate capability of olivine LiCoPO 4 , Cr, Fe, and Si were added to produce nominal Li 1.025 Co 0.84 Fe 0.10 Cr 0.05 Si 0.01 (PO 4) 1.025. This cathode material has comparable energy density to LiCoPO , with markedly improved electrochemical performance. Here we apply operando X-ray diffraction to gain understanding of the unique crystallographic delithiation mechanism of this new substituted electrode material. Throughout charging, the extent of solid solution domains was significantly increased in Li 1.025 Co 0.84 Fe 0.10 Cr 0.05 Si 0.01 (PO 4) 1.025 and LiCo 0.75 Fe 0.25 PO 4 compared to LiCoPO 4. These domains reduce mechanical strain during electrode function, providing a clear explanation for the high durability with Co substitution. Li 1.025 Co 0.84 Fe 0.10 Cr 0.05 Si 0.01 (PO 4) 1.025 operated at notably higher average potential than LiCo 0.75 Fe 0.25 PO 4 , which would increase the energy density of the cell. Ex situ measurements reveal the persistence of structural irreversibilities in the substituted phase after the first cycle, identifying avenues for further improvement in durability. This finding sheds light into strategies for judicious cation substitution in LiCoPO 4 electrodes to maximize cycle life while preserving high energy density, especially compared to LiFePO 4 .

ECS Meeting Abstracts, 2017

Next generation lithium ion batteries need higher energy density to displace the current technolo... more Next generation lithium ion batteries need higher energy density to displace the current technology, which operate around 4 V. Higher energy density can be achieved by increasing the specific capacity and/or the working voltage of the cathode active material. One such possibility is LiCoPO4 (LCP) which theoretically delivers 167 mAh g-1 at 4.8 V. Unfortunately, LCP undergoes rapid capacity fading in its pure form. The introduction of small quantities of dopants to create a substituted-LCP has greatly increased the cycle life over pure LCP [1]. With the development of new high voltage cathode materials such as LCP comes the need for electrolytes that will enable their implementation. Current state of the art (SOA) LiPF6/carbonate based electrolytes are only stable up to ~4.5 V. Two strategies used to increase the electrochemical window of electrolytes include using different solvents such as fluorinated solvents that are stable to higher voltages or using sacrificial additives that u...

ECS Meeting Abstracts, 2017

ECS Meeting Abstracts, 2016

There is an ever-growing need for higher energy Li-ion batteries. Lithium metal phospho-olivines ... more There is an ever-growing need for higher energy Li-ion batteries. Lithium metal phospho-olivines based on LiCoPO4 are promising cathode materials to help fulfill this need owing to their relatively high discharge capacity of up to 167 mAh g-1 at a discharge potential of 4.8V [1]. However, LiCoPO4 based Li-ion cells show a severe loss of discharge capacity upon multiple charge – discharge cycles owing to structure deterioration and electrolyte decomposition [2]. Partial substitution of Co by Fe significantly improves the cycle life [3] and increases Li+ ion and electrical transport [4]. Our most recent studies show that multi-element substitution for Co in LiCoPO4 further reduces capacity fade, substantially improves discharge capacity and reduces reactivity of the cathode with the electrolyte. Currently, we have achieved an energy storage capacity of 670 Wh kg-1 of cathode material (84% of LiCoPO4 theoretical) which can be compared to a maximum theoretical 576 Wh kg-1 for commercial...

ECS Meeting Abstracts, 2014

ECS Meeting Abstracts, 2013

Journal of the American Ceramic Society, 2016

The effect of relative density on the hardness and fracture toughness of Al-substituted cubic gar... more The effect of relative density on the hardness and fracture toughness of Al-substituted cubic garnet Li 6.19 Al 0.27 La 3 Zr 2 O 12 (LLZO) was investigated. Polycrystalline LLZO was made using solid-state synthesis and hot-pressing. The relative density was controlled by varying the densification time at fixed temperature (1050°C) and pressure (62 MPa). After hotpressing, the average grain size varied from approximately 2.7-3.7 lm for the 85% and 98% relative density samples, respectively. Examination of fracture surfaces revealed a transition from inter-to intragranular fracture as the relative density increased. The Vickers hardness increased with relative density up to 96%, above which the hardness was constant. At 98% relative density, the Vickers hardness was equal to the hardness measured by nanoindentation 9.1 GPa, which is estimated as the single-crystal hardness value. An inverse correlation between relative density and fracture toughness was observed. The fracture toughness increased linearly from 0.97 to 2.37 MPa√m for the 98% and 85% relative density samples, respectively. It is suggested that crack deflection along grain boundaries can explain the increase in fracture toughness with decreasing relative density. It was also observed that the total ionic conductivity increased from 0.0094 to 0.34 mS/cm for the 85%-98% relative density samples, respectively. The results of this study suggest that the microstructure of LLZO must be optimized to maximize mechanical integrity and ionic conductivity.

Chemistry of Materials, 2015

The oxide garnet material Li7La3Zr2O12 shows remarkably high ionic conductivity when doped with s... more The oxide garnet material Li7La3Zr2O12 shows remarkably high ionic conductivity when doped with supervalent ions that are charge compensated by Li vacancies and is currently one of the best candidates for development of a technologically relevant solid electrolyte. Determination of optimal dopant concentration, however, has remained a persistent problem due to the extreme difficulty of establishing the actual (as compared to nominal) stoichiometry of intentionally doped materials and by the fact that it is still not entirely clear what level of lattice expansion/contraction best promotes ionic diffusion. By combining careful synthesis, neutron diffraction, high-resolution X-ray diffraction (XRD), Raman measurements, and density functional theory calculations, we show that structure and stoichiometry are intimately related such that the former can in many cases be used as a gauge of the latter. We show that different Li-vacancy creating supervalent ions (Al3+ vs Ta5+) affect the structure very differently,...

The Journal of Physical Chemistry B, 1999

Electrochemical doping of bisulfate ions into single wall carbon nanotube (SWNT) bundles has been... more Electrochemical doping of bisulfate ions into single wall carbon nanotube (SWNT) bundles has been studied using coulometry, cyclic voltammetry, mass-uptake measurements, and Raman scattering experiments. A spontaneous charge-transfer reaction is observed prior to the application of an electrochemical driving force, in sharp contrast to previous observations in the graphite-H 2 SO 4 system. A mass increase of the SWNT sample and a concomitant upshift of the Raman-active tangential mode frequency indicate oxidation (i.e., removal of electrons) of the SWNT bundles. In fact, using Raman scattering, we were able to separate the spontaneous and electrochemical contributions to the overall charge transfer, resulting in the value of an upshift of 320 cm-1 per hole, per C-atom introduced into the carbon π-band by the bisulfate (HSO 4-) dopant. This value may prove to be a universal measure of charge transfer in acceptor-type SWNT compounds. At a critical electrochemical doping, the SWNT bundles are driven into an "overoxidation" regime, where they are irreversibly oxidized with the formation of CO covalent bonds, analogous to electrochemical formation of graphite oxides.

Molecules, 2021

Spinel-structured solids were studied to understand if fast Li+ ion conduction can be achieved wi... more Spinel-structured solids were studied to understand if fast Li+ ion conduction can be achieved with Li occupying multiple crystallographic sites of the structure to form a “Li-stuffed” spinel, and if the concept is applicable to prepare a high mixed electronic-ionic conductive, electrochemically active solid solution of the Li+ stuffed spinel with spinel-structured Li-ion battery electrodes. This could enable a single-phase fully solid electrode eliminating multi-phase interface incompatibility and impedance commonly observed in multi-phase solid electrolyte–cathode composites. Materials of composition Li1.25M(III)0.25TiO4, M(III) = Cr or Al were prepared through solid-state methods. The room-temperature bulk Li+-ion conductivity is 1.63 × 10−4 S cm−1 for the composition Li1.25Cr0.25Ti1.5O4. Addition of Li3BO3 (LBO) increases ionic and electronic conductivity reaching a bulk Li+ ion conductivity averaging 6.8 × 10−4 S cm−1, a total Li-ion conductivity averaging 4.2 × 10−4 S cm−1, an...

Ionics, 2017

Li-ion-conducting solid electrolytes are receiving considerable attention for use in advanced bat... more Li-ion-conducting solid electrolytes are receiving considerable attention for use in advanced batteries. These electrolytes would enable use of a Li metal anode, allowing for batteries with higher energy densities and enhanced safety compared to current Liion systems. One important aspect of these electrolytes that has been overlooked is their mechanical properties. Mechanical properties will play a large role in the processing, assembly, and operation of battery cells. Hence, this paper reviews the elastic, plastic, and fracture properties of crystalline oxide-based Li-ion solid electrolytes for three different crystal structures: Li 6.19 Al 0.27 La 3 Zr 2 O 12 (garnet) [LLZO], Li 0.33 La 0.57 TiO 3 (perovskite) [LLTO], and Li 1.3 Al 0.3 Ti 1.7 (PO 4) 3 (NaSICON) [LATP]. The experimental Young's modulus value for (1) LLTO is~200 GPa, (2) LLZO is~150 GPa, and (3) for LATP~115 GPa. The experimental values are in good agreement with density functional theory predictions. The fracture toughness value for all three of LLTO, LLZO, and LATP is approximately 1 MPa m −2. This low value is expected since, they all exhibit at least some degree of covalent bonding, which limits dislocation mobility leading to brittle behavior.

ACS Energy Letters, 2017

The recent discovery of fast ion-conducting solid electrolytes could enable solid-state and other... more The recent discovery of fast ion-conducting solid electrolytes could enable solid-state and other advanced battery chemistries with higher energy densities and enhanced safety. In addition to high ionic conductivity, a viable electrolyte should also exhibit an electrochemical window that is wide enough to suppress undesirable electronic transport (i.e., self-discharge and/ or short circuiting) arising from charge injection or extraction from the electrodes. Here, direct current chronoamperometry, alternating current electrochemical impedance spectroscopy, and optical absorption band gap measurements are combined with first-principles calculations to systematically characterize the electrochemical window of the promising superionic conductor Li 7 La 3 Zr 2 O 12 (LLZO). Negligible electronic current was measured within LLZO for a wide range of voltages relevant for high-voltage cathodes. This auspicious behavior is consistent with both the large band gap (∼6 eV) predicted for LLZO and the absolute positions of its band edges. These features imply that a wide electrochemical window is an intrinsic property of LLZO, facilitating its use in next-generation batteries.

Solid State Ionics, 2017

The elastic modulus, fracture toughness, ionic conductivity, thermal expansion, Li and water stab... more The elastic modulus, fracture toughness, ionic conductivity, thermal expansion, Li and water stability of dense (∼97%) fine-grain (∼10 µm) Li 1.2 Zr 1.9 Sr 0.1 (PO 4) 3 (LZSP)consolidated by reactive hot-pressing was investigated. Low values of elastic modulus (25.2 and 40.9 GPa), fracture toughness (0.29 and 0.37 MPa-m 1/2), total conductivity (2.1 x 10-5 S/cm) and thermal expansion hysteresis were exhibited as result of the presence of microcracks formed during cooling from the hot-pressing temperature. The lattice conductivity of LZSP is 0.85 x 10-4 S/cm with an activation energy for bulk conductivity of 0.29 eV. The results of Li/LZSP/Li cell impedance as a function of time and surface examination suggest that LZSP is not stable against metallic lithium. The results of the water stability test suggest that LZSP is a possible membrane for use in aqueous Li-Air cells. The results of this study reveal for the case of lithium zirconium phosphates are as follows: 1] a small grain size is required to prevent microcracking to avoid a reduction in total conductivity and mechanical properties and 2] even though they do not contain a transition metal cation they are not stable against Li at room temperature.

ChemElectroChem

The synthesis and electrochemical insertion of lithium into the Wadsley–Roth NaNb13O33 phase is s... more The synthesis and electrochemical insertion of lithium into the Wadsley–Roth NaNb13O33 phase is studied. Lithium intercalation to form LixNaNb13O33 reaches a value of up to x~15, between 3.0 and 1.0 V vs. Li+/Li at a slow cycling rate, a capacity of 233 mAh g−1. Within this voltage window, two sharp peaks and one broad peak are observed in the differential capacity plots of lithium intercalation suggesting multiple two‐phase regions. High Li‐ion conductivity and rate capability was demonstrated. The lithium diffusion constant is about an order of magnitude greater than TiNb2O7. The average voltage is about 1.6 V and its high‐rate capability makes NaNb13O33 potentially useful as an anode in a fast‐charge Li‐ion battery application.

ECS Meeting Abstracts

Unlike batteries in classical experimental settings, batteries in practical applications are gene... more Unlike batteries in classical experimental settings, batteries in practical applications are generally not discharged immediately upon reaching a fully charged state but remain for a period of time before usage. Such a state is demanding on the cathode as much of the Ni-rich layered oxide degradation mechanisms occur at the deeply charged state. Here, we examine a Ni-rich cathode material (i.e., Li[Ni0.90Co0.05Mn0.05]O2) under conditions simulating real-use behavior of batteries and find that the addition of a short dwell period at the deeply charged state leads to substantial differences in cycling performance (89.4 % versus 37.5 % retention rates after 100 cycles, respectively). Furthermore, to overcome the rapid deterioration, Sb was used as a dopant to promote stability, especially at the grain boundaries regions, to suppress degradation at the cathode-electrolyte interface. The resulting cathode energy material (i.e., Li[Ni0.895Co0.05Mn0.05Sb0.05]O2) proved to be stable during ...

ECS Meeting Abstracts, 2016

The energy storage capability of a battery scales with the potential difference between its elect... more The energy storage capability of a battery scales with the potential difference between its electrodes. Yet operation of positive electrode materials at high potentials introduces challenges of stabilization of charged states. As of today, no positive electrode material has been demonstrated to durably and safely operate above 4.5 V vs. Li+/Li0. LiCoPO4-based electrodes theoretically offer high specific capacity and high potentials of operation, around 4.8V vs. Li+/Li0, but these electrodes are prone to failure during cycling. Failure occurs through chemical and structural degradation in the bulk of the active material or at its interfaces with cell components, especially the electrolyte. The development of Li-ion battery electrodes operating at high potential is indispensable to meet the specific energy target of 250 kWh/kg at the packaged cell level. Changes in the electronic structure and chemical stability of olivine-type LiCoPO4 and Fe-substituted LiCoPO4 were explored as both ...

ECS Meeting Abstracts

Solid Li-ion conducting electrolytes are one pathway towards future, energy-dense, intrinsically-... more Solid Li-ion conducting electrolytes are one pathway towards future, energy-dense, intrinsically-safe solid-state batteries. Thus, it is of interest to study novel fast Li-ion solid electrolyte materials. Here we report the synthesis and characterization of a family of spinel structured oxide solid electrolytes[1]. Further, we will report on the interface and mixed electronic-ionic conductivity of the spinel structured solid solution which is formed upon reaction with spinel structured Li-ion battery electrode materials. We detail the compositions that were explored and give the results of the synthesis, densification and the structural, physical and electrochemical characterization. The properties of the new materials will be compared and contrasted to well-known solid Li electrolytes such as garnet and NASICON structured materials. References 1. Allen, J.L.; Crear, B.A.; Choudhury, R.; Wang, M.J.; Tran, D.T.; Ma, L.; Piccoli, P.M.; Sakamoto, J.; Wolfenstine, J. Fast Li-Ion Conduct...

Electrochemical and Solid-State Letters, 2003

The electrochemical intercalation of unsolvated lithium cations into polyparaphenylene (PPP) has ... more The electrochemical intercalation of unsolvated lithium cations into polyparaphenylene (PPP) has been carried out in a EC/PC (1/1 volume) mixture using LiClO 4 as lithium salt. It leads to formation of saturated materials with composition Li 0.5 (C 6 H 4). Cyclic voltammetry and galvanostatic measurements reveal three successive phase transformations in the intercalation-deintercalation process. The irreversible side reactions which occur during the first PPP reduction, like decomposition of the electrolytes (EC and PC) and reduction of the binder (PVDF), are responsible for an important capacity loss that decreases rapidly in the following cycles. The cells exhibit finally a good reversibility.

ACS Applied Materials & Interfaces, 2020

In an effort to improve the cycle life and rate capability of olivine LiCoPO 4 , Cr, Fe, and Si w... more In an effort to improve the cycle life and rate capability of olivine LiCoPO 4 , Cr, Fe, and Si were added to produce nominal Li 1.025 Co 0.84 Fe 0.10 Cr 0.05 Si 0.01 (PO 4) 1.025. This cathode material has comparable energy density to LiCoPO , with markedly improved electrochemical performance. Here we apply operando X-ray diffraction to gain understanding of the unique crystallographic delithiation mechanism of this new substituted electrode material. Throughout charging, the extent of solid solution domains was significantly increased in Li 1.025 Co 0.84 Fe 0.10 Cr 0.05 Si 0.01 (PO 4) 1.025 and LiCo 0.75 Fe 0.25 PO 4 compared to LiCoPO 4. These domains reduce mechanical strain during electrode function, providing a clear explanation for the high durability with Co substitution. Li 1.025 Co 0.84 Fe 0.10 Cr 0.05 Si 0.01 (PO 4) 1.025 operated at notably higher average potential than LiCo 0.75 Fe 0.25 PO 4 , which would increase the energy density of the cell. Ex situ measurements reveal the persistence of structural irreversibilities in the substituted phase after the first cycle, identifying avenues for further improvement in durability. This finding sheds light into strategies for judicious cation substitution in LiCoPO 4 electrodes to maximize cycle life while preserving high energy density, especially compared to LiFePO 4 .

ECS Meeting Abstracts, 2017

Next generation lithium ion batteries need higher energy density to displace the current technolo... more Next generation lithium ion batteries need higher energy density to displace the current technology, which operate around 4 V. Higher energy density can be achieved by increasing the specific capacity and/or the working voltage of the cathode active material. One such possibility is LiCoPO4 (LCP) which theoretically delivers 167 mAh g-1 at 4.8 V. Unfortunately, LCP undergoes rapid capacity fading in its pure form. The introduction of small quantities of dopants to create a substituted-LCP has greatly increased the cycle life over pure LCP [1]. With the development of new high voltage cathode materials such as LCP comes the need for electrolytes that will enable their implementation. Current state of the art (SOA) LiPF6/carbonate based electrolytes are only stable up to ~4.5 V. Two strategies used to increase the electrochemical window of electrolytes include using different solvents such as fluorinated solvents that are stable to higher voltages or using sacrificial additives that u...

ECS Meeting Abstracts, 2017

ECS Meeting Abstracts, 2016

There is an ever-growing need for higher energy Li-ion batteries. Lithium metal phospho-olivines ... more There is an ever-growing need for higher energy Li-ion batteries. Lithium metal phospho-olivines based on LiCoPO4 are promising cathode materials to help fulfill this need owing to their relatively high discharge capacity of up to 167 mAh g-1 at a discharge potential of 4.8V [1]. However, LiCoPO4 based Li-ion cells show a severe loss of discharge capacity upon multiple charge – discharge cycles owing to structure deterioration and electrolyte decomposition [2]. Partial substitution of Co by Fe significantly improves the cycle life [3] and increases Li+ ion and electrical transport [4]. Our most recent studies show that multi-element substitution for Co in LiCoPO4 further reduces capacity fade, substantially improves discharge capacity and reduces reactivity of the cathode with the electrolyte. Currently, we have achieved an energy storage capacity of 670 Wh kg-1 of cathode material (84% of LiCoPO4 theoretical) which can be compared to a maximum theoretical 576 Wh kg-1 for commercial...

ECS Meeting Abstracts, 2014

ECS Meeting Abstracts, 2013

Journal of the American Ceramic Society, 2016

The effect of relative density on the hardness and fracture toughness of Al-substituted cubic gar... more The effect of relative density on the hardness and fracture toughness of Al-substituted cubic garnet Li 6.19 Al 0.27 La 3 Zr 2 O 12 (LLZO) was investigated. Polycrystalline LLZO was made using solid-state synthesis and hot-pressing. The relative density was controlled by varying the densification time at fixed temperature (1050°C) and pressure (62 MPa). After hotpressing, the average grain size varied from approximately 2.7-3.7 lm for the 85% and 98% relative density samples, respectively. Examination of fracture surfaces revealed a transition from inter-to intragranular fracture as the relative density increased. The Vickers hardness increased with relative density up to 96%, above which the hardness was constant. At 98% relative density, the Vickers hardness was equal to the hardness measured by nanoindentation 9.1 GPa, which is estimated as the single-crystal hardness value. An inverse correlation between relative density and fracture toughness was observed. The fracture toughness increased linearly from 0.97 to 2.37 MPa√m for the 98% and 85% relative density samples, respectively. It is suggested that crack deflection along grain boundaries can explain the increase in fracture toughness with decreasing relative density. It was also observed that the total ionic conductivity increased from 0.0094 to 0.34 mS/cm for the 85%-98% relative density samples, respectively. The results of this study suggest that the microstructure of LLZO must be optimized to maximize mechanical integrity and ionic conductivity.

Chemistry of Materials, 2015

The oxide garnet material Li7La3Zr2O12 shows remarkably high ionic conductivity when doped with s... more The oxide garnet material Li7La3Zr2O12 shows remarkably high ionic conductivity when doped with supervalent ions that are charge compensated by Li vacancies and is currently one of the best candidates for development of a technologically relevant solid electrolyte. Determination of optimal dopant concentration, however, has remained a persistent problem due to the extreme difficulty of establishing the actual (as compared to nominal) stoichiometry of intentionally doped materials and by the fact that it is still not entirely clear what level of lattice expansion/contraction best promotes ionic diffusion. By combining careful synthesis, neutron diffraction, high-resolution X-ray diffraction (XRD), Raman measurements, and density functional theory calculations, we show that structure and stoichiometry are intimately related such that the former can in many cases be used as a gauge of the latter. We show that different Li-vacancy creating supervalent ions (Al3+ vs Ta5+) affect the structure very differently,...

The Journal of Physical Chemistry B, 1999

Electrochemical doping of bisulfate ions into single wall carbon nanotube (SWNT) bundles has been... more Electrochemical doping of bisulfate ions into single wall carbon nanotube (SWNT) bundles has been studied using coulometry, cyclic voltammetry, mass-uptake measurements, and Raman scattering experiments. A spontaneous charge-transfer reaction is observed prior to the application of an electrochemical driving force, in sharp contrast to previous observations in the graphite-H 2 SO 4 system. A mass increase of the SWNT sample and a concomitant upshift of the Raman-active tangential mode frequency indicate oxidation (i.e., removal of electrons) of the SWNT bundles. In fact, using Raman scattering, we were able to separate the spontaneous and electrochemical contributions to the overall charge transfer, resulting in the value of an upshift of 320 cm-1 per hole, per C-atom introduced into the carbon π-band by the bisulfate (HSO 4-) dopant. This value may prove to be a universal measure of charge transfer in acceptor-type SWNT compounds. At a critical electrochemical doping, the SWNT bundles are driven into an "overoxidation" regime, where they are irreversibly oxidized with the formation of CO covalent bonds, analogous to electrochemical formation of graphite oxides.

Molecules, 2021

Spinel-structured solids were studied to understand if fast Li+ ion conduction can be achieved wi... more Spinel-structured solids were studied to understand if fast Li+ ion conduction can be achieved with Li occupying multiple crystallographic sites of the structure to form a “Li-stuffed” spinel, and if the concept is applicable to prepare a high mixed electronic-ionic conductive, electrochemically active solid solution of the Li+ stuffed spinel with spinel-structured Li-ion battery electrodes. This could enable a single-phase fully solid electrode eliminating multi-phase interface incompatibility and impedance commonly observed in multi-phase solid electrolyte–cathode composites. Materials of composition Li1.25M(III)0.25TiO4, M(III) = Cr or Al were prepared through solid-state methods. The room-temperature bulk Li+-ion conductivity is 1.63 × 10−4 S cm−1 for the composition Li1.25Cr0.25Ti1.5O4. Addition of Li3BO3 (LBO) increases ionic and electronic conductivity reaching a bulk Li+ ion conductivity averaging 6.8 × 10−4 S cm−1, a total Li-ion conductivity averaging 4.2 × 10−4 S cm−1, an...

Ionics, 2017

Li-ion-conducting solid electrolytes are receiving considerable attention for use in advanced bat... more Li-ion-conducting solid electrolytes are receiving considerable attention for use in advanced batteries. These electrolytes would enable use of a Li metal anode, allowing for batteries with higher energy densities and enhanced safety compared to current Liion systems. One important aspect of these electrolytes that has been overlooked is their mechanical properties. Mechanical properties will play a large role in the processing, assembly, and operation of battery cells. Hence, this paper reviews the elastic, plastic, and fracture properties of crystalline oxide-based Li-ion solid electrolytes for three different crystal structures: Li 6.19 Al 0.27 La 3 Zr 2 O 12 (garnet) [LLZO], Li 0.33 La 0.57 TiO 3 (perovskite) [LLTO], and Li 1.3 Al 0.3 Ti 1.7 (PO 4) 3 (NaSICON) [LATP]. The experimental Young's modulus value for (1) LLTO is~200 GPa, (2) LLZO is~150 GPa, and (3) for LATP~115 GPa. The experimental values are in good agreement with density functional theory predictions. The fracture toughness value for all three of LLTO, LLZO, and LATP is approximately 1 MPa m −2. This low value is expected since, they all exhibit at least some degree of covalent bonding, which limits dislocation mobility leading to brittle behavior.

ACS Energy Letters, 2017

The recent discovery of fast ion-conducting solid electrolytes could enable solid-state and other... more The recent discovery of fast ion-conducting solid electrolytes could enable solid-state and other advanced battery chemistries with higher energy densities and enhanced safety. In addition to high ionic conductivity, a viable electrolyte should also exhibit an electrochemical window that is wide enough to suppress undesirable electronic transport (i.e., self-discharge and/ or short circuiting) arising from charge injection or extraction from the electrodes. Here, direct current chronoamperometry, alternating current electrochemical impedance spectroscopy, and optical absorption band gap measurements are combined with first-principles calculations to systematically characterize the electrochemical window of the promising superionic conductor Li 7 La 3 Zr 2 O 12 (LLZO). Negligible electronic current was measured within LLZO for a wide range of voltages relevant for high-voltage cathodes. This auspicious behavior is consistent with both the large band gap (∼6 eV) predicted for LLZO and the absolute positions of its band edges. These features imply that a wide electrochemical window is an intrinsic property of LLZO, facilitating its use in next-generation batteries.

Solid State Ionics, 2017

The elastic modulus, fracture toughness, ionic conductivity, thermal expansion, Li and water stab... more The elastic modulus, fracture toughness, ionic conductivity, thermal expansion, Li and water stability of dense (∼97%) fine-grain (∼10 µm) Li 1.2 Zr 1.9 Sr 0.1 (PO 4) 3 (LZSP)consolidated by reactive hot-pressing was investigated. Low values of elastic modulus (25.2 and 40.9 GPa), fracture toughness (0.29 and 0.37 MPa-m 1/2), total conductivity (2.1 x 10-5 S/cm) and thermal expansion hysteresis were exhibited as result of the presence of microcracks formed during cooling from the hot-pressing temperature. The lattice conductivity of LZSP is 0.85 x 10-4 S/cm with an activation energy for bulk conductivity of 0.29 eV. The results of Li/LZSP/Li cell impedance as a function of time and surface examination suggest that LZSP is not stable against metallic lithium. The results of the water stability test suggest that LZSP is a possible membrane for use in aqueous Li-Air cells. The results of this study reveal for the case of lithium zirconium phosphates are as follows: 1] a small grain size is required to prevent microcracking to avoid a reduction in total conductivity and mechanical properties and 2] even though they do not contain a transition metal cation they are not stable against Li at room temperature.