Ronald Väli | University of Tartu (original) (raw)

Papers by Ronald Väli

Journal of The Electrochemical Society, 2020

Voltage fade during charge-discharge cycling in Layered Li-rich Mn-rich positive electrode materi... more Voltage fade during charge-discharge cycling in Layered Li-rich Mn-rich positive electrode materials needs to be overcome for the development of high-energy low cost Li-ion batteries. Several cation dopants have been introduced into the bulk lattice to mitigate voltage decay by limiting transition metal (TM) migration, inhibiting phase transformation, or reducing the extent of oxygen release. Here, a series of electrochemically active Cr substituted (2.5, 5.0, and 10 mol%) Co-free Li1.11Ni0.33Mn0.56O2 and Li1.2Ni0.2Mn0.6O2 compositions were synthesized via dry particle fusion followed by heat treatment with Li2CO3. Cr doping improves specific capacity and capacity retention via multiple electron transfer of Cr3+/Cr6+ as well as mitigates voltage fading to a certain extent. The impact of Cr on voltage decay was studied by careful measurements of dQ/dV vs V on Cr-doped and undoped samples before and after cycle testing.

Journal of The Electrochemical Society

Batteries, 2019

Cost-effective methods need to be developed to lower the price of Na-ion battery (NIB) materials.... more Cost-effective methods need to be developed to lower the price of Na-ion battery (NIB) materials. This paper reports a proof-of-concept study of using a novel approach to the glycine-nitrate process (GNP) to synthesize sodium vanadium phosphate (Na 3 V 2 (PO 4) 3 or NVP) materials with both high-energy (102 mAh g −1 at C/20) and high-power characteristics (60 mAh g −1 at 20 C). Glucose-derived hard carbons (GDHCs) were optimized to reduce both sloping and irreversible capacity. The best results were achieved for electrodes with active material heat treated at 1400 • C and reduced Super P additive. Sloping region capacity 90 mAh g −1 , irreversible capacity 47 mAh g −1 , discharge capacity 272 mAh g −1 (of which plateau 155 mAh g −1) and 1st cycle coulombic efficiency (CE) 85% were demonstrated. GDHC||NVP full cell achieved 80 mAh g −1 (reversible) by NVP mass out of which 60 mAh g −1 was the plateau (3.4 V) region capacity. Full cell specific energy and energy density reached 189 Wh kg −1 and 104 Wh dm −3 , respectively. After 80 cycles, including rate testing from C/20 to 10 C, the cell cycled at 65 mAh g −1 with 99.7% CE. With further optimization, this method can have very high industrial potential.

The electrochemical performance of glucose-derived hard carbon (GDHC) anode has been evaluated us... more The electrochemical performance of glucose-derived hard carbon (GDHC) anode has been evaluated using Li-and Na-salts in ethylene carbonate and propylene carbonate electrolyte mixtures. The LiPF 6 /EC: PC (1:1) system exhibits high capacity at low current densities (400 mAh/g at 25 mA/g) and also good power characteristics retaining 150 mAh/g capacity at 2 A/g current density. The best overall performance was achieved with 1 M NaPF 6 /EC:PC (1:1) electrolyte based system with capacities of 175 mAh/g at 0.1 V vs Na/Na + and 330 mAh/g at 1.5 V vs Na/Na+. The electrode has been physically characterized ex-situ using SEM, Raman and TOF-SIMS methods TOF-SIMS analysis revealed that the solid electrolyte interphase is more inorganic on the negative electrode in the Na-cell than on the negative electrode the Li-cell. The positive ion-specific images established by TOF-SIMS analysis show the non-homogeneous distribution of various fragments from the pristine GDHC, which is caused by slightly inhomogeneous mixture of GDHC and conducting carbon black (Super P) particles.

In this study, we report the results of electrochemical impedance spectroscopy data modelling of ... more In this study, we report the results of electrochemical impedance spectroscopy data modelling of various battery half-cells with different alkali metal (Li, Na, K) salts. Test results of electrochemical half-cells were evaluated for the D-glucose derived hard carbon negative electrode in 1.0 M LiPF 6 + EC:DMC (1:1 volume ratio), 1.0 M NaPF 6 + EC:DMC (1:1), 1.0 M NaClO 4 + PC, 0.8 M KPF 6 + EC:DEC (1:1) and 0.8 M KPF 6 + EC:DMC (1:1) solutions at 0.5 mV s −1 potential scan rate measured within the potential region from 0.05 V to 1.2 V (vs Me/Me + ) (where Me is Li, Na or K). Modelling of electrochemical impedance spectroscopy data was employed to characterize alkali metal insertion processes in/on D-glucose derived hard carbon anode. Detailed analysis of impedance data shows that Newman equivalent circuit modified with a constant phase element can be applied for calculation of impedance spectra and fitting of calculated data to experimental ones, using non-linear least square root fitting method. Equivalent circuit fit parameters depend strongly on electrolyte composition. Very slow processes have been observed for KPF 6 + EC:DEC based half-cell. Comparatively quick metal-cation reduction and accumulation processes have been observed in NaClO 4 + PC and LiPF 6 +EC:DMC based half-cell anodes.

Electrochemical performance of nanospheric glucose derived hard carbon based electrodes, speciall... more Electrochemical performance of nanospheric glucose derived hard carbon based electrodes, specially derived from D-glucose via hydrothermal carbonization and subsequent pyrolysis at 1100◦C, has been studied in 1 M NaClO4 propylene carbonate electrolyte. High Na+
electroreduction and Na oxidation peaks were observed in cyclic voltammograms. Galvanostatic charge/discharge measurements demonstrated high specific capacity, over 300 mAh g−1 for the first cycle. After 200thcycle, a specific capacity of 160 mAh g−1(at 50 mA g−1 cycling rate) has been calculated for the nanospheric hard carbon based half-cells. Raman spectroscopy, inductively coupled plasma mass spectrometry and energy-dispersive X-ray spectroscopy data indicated accumulation (adsorption/absorption) of Na onto/into the electrochemically polarized porous nanospheric hard carbon material. Impedance data demonstrated that electrode potential has a noticeable influence on the total polarization, and charge transfer resistance as well as on the mass transfer characteristics at the carbon | (1 M NaClO4 + propylene carbonate) electrolyte interface.

The electrochemical characteristics of supercapacitors consisting of molybdenum carbide derived c... more The electrochemical characteristics of supercapacitors consisting of molybdenum carbide derived carbon C(Mo2C) electrodes in 1M NaPF 6 solutions in various mixtures (0.5–5%) of vinylene carbonate (VC) with propylene carbonate (PC) and ethyl acetate (EA) (1:1 by volume) have been studied using cyclic voltammetry, constant current charging/discharging and electrochemical impedance spectroscopy methods. The specific capacitance and series resistance values dependencies on the used solvent mixture and applied temperature (from −40 • C to 60 • C) have been established. The region of ideal polarizability has been established for C(Mo 2 C) electrodes in all electrolytes and temperatures investigated, except at T ≥ 40 • C. Specific conductivity values have been measured and correlated with electrochemistry data. Limiting capacitance, calculated characteristic time constant and complex power values depend noticeably on the solvent mixture used in the electrolyte, i.e. on the viscosity and specific conductivity of the electrolyte solution. The studied electrolytes are potential candidates for low-temperature supercapacitors.

Na-salt non-aqueous electrolyte Low-temperature supercapacitor Carbide derived carbon Solvent mix... more Na-salt non-aqueous electrolyte Low-temperature supercapacitor Carbide derived carbon Solvent mixture for wide temperature range a b s t r a c t

Journal of The Electrochemical Society, 2020

Voltage fade during charge-discharge cycling in Layered Li-rich Mn-rich positive electrode materi... more Voltage fade during charge-discharge cycling in Layered Li-rich Mn-rich positive electrode materials needs to be overcome for the development of high-energy low cost Li-ion batteries. Several cation dopants have been introduced into the bulk lattice to mitigate voltage decay by limiting transition metal (TM) migration, inhibiting phase transformation, or reducing the extent of oxygen release. Here, a series of electrochemically active Cr substituted (2.5, 5.0, and 10 mol%) Co-free Li1.11Ni0.33Mn0.56O2 and Li1.2Ni0.2Mn0.6O2 compositions were synthesized via dry particle fusion followed by heat treatment with Li2CO3. Cr doping improves specific capacity and capacity retention via multiple electron transfer of Cr3+/Cr6+ as well as mitigates voltage fading to a certain extent. The impact of Cr on voltage decay was studied by careful measurements of dQ/dV vs V on Cr-doped and undoped samples before and after cycle testing.

Journal of The Electrochemical Society

Batteries, 2019

Cost-effective methods need to be developed to lower the price of Na-ion battery (NIB) materials.... more Cost-effective methods need to be developed to lower the price of Na-ion battery (NIB) materials. This paper reports a proof-of-concept study of using a novel approach to the glycine-nitrate process (GNP) to synthesize sodium vanadium phosphate (Na 3 V 2 (PO 4) 3 or NVP) materials with both high-energy (102 mAh g −1 at C/20) and high-power characteristics (60 mAh g −1 at 20 C). Glucose-derived hard carbons (GDHCs) were optimized to reduce both sloping and irreversible capacity. The best results were achieved for electrodes with active material heat treated at 1400 • C and reduced Super P additive. Sloping region capacity 90 mAh g −1 , irreversible capacity 47 mAh g −1 , discharge capacity 272 mAh g −1 (of which plateau 155 mAh g −1) and 1st cycle coulombic efficiency (CE) 85% were demonstrated. GDHC||NVP full cell achieved 80 mAh g −1 (reversible) by NVP mass out of which 60 mAh g −1 was the plateau (3.4 V) region capacity. Full cell specific energy and energy density reached 189 Wh kg −1 and 104 Wh dm −3 , respectively. After 80 cycles, including rate testing from C/20 to 10 C, the cell cycled at 65 mAh g −1 with 99.7% CE. With further optimization, this method can have very high industrial potential.

The electrochemical performance of glucose-derived hard carbon (GDHC) anode has been evaluated us... more The electrochemical performance of glucose-derived hard carbon (GDHC) anode has been evaluated using Li-and Na-salts in ethylene carbonate and propylene carbonate electrolyte mixtures. The LiPF 6 /EC: PC (1:1) system exhibits high capacity at low current densities (400 mAh/g at 25 mA/g) and also good power characteristics retaining 150 mAh/g capacity at 2 A/g current density. The best overall performance was achieved with 1 M NaPF 6 /EC:PC (1:1) electrolyte based system with capacities of 175 mAh/g at 0.1 V vs Na/Na + and 330 mAh/g at 1.5 V vs Na/Na+. The electrode has been physically characterized ex-situ using SEM, Raman and TOF-SIMS methods TOF-SIMS analysis revealed that the solid electrolyte interphase is more inorganic on the negative electrode in the Na-cell than on the negative electrode the Li-cell. The positive ion-specific images established by TOF-SIMS analysis show the non-homogeneous distribution of various fragments from the pristine GDHC, which is caused by slightly inhomogeneous mixture of GDHC and conducting carbon black (Super P) particles.

In this study, we report the results of electrochemical impedance spectroscopy data modelling of ... more In this study, we report the results of electrochemical impedance spectroscopy data modelling of various battery half-cells with different alkali metal (Li, Na, K) salts. Test results of electrochemical half-cells were evaluated for the D-glucose derived hard carbon negative electrode in 1.0 M LiPF 6 + EC:DMC (1:1 volume ratio), 1.0 M NaPF 6 + EC:DMC (1:1), 1.0 M NaClO 4 + PC, 0.8 M KPF 6 + EC:DEC (1:1) and 0.8 M KPF 6 + EC:DMC (1:1) solutions at 0.5 mV s −1 potential scan rate measured within the potential region from 0.05 V to 1.2 V (vs Me/Me + ) (where Me is Li, Na or K). Modelling of electrochemical impedance spectroscopy data was employed to characterize alkali metal insertion processes in/on D-glucose derived hard carbon anode. Detailed analysis of impedance data shows that Newman equivalent circuit modified with a constant phase element can be applied for calculation of impedance spectra and fitting of calculated data to experimental ones, using non-linear least square root fitting method. Equivalent circuit fit parameters depend strongly on electrolyte composition. Very slow processes have been observed for KPF 6 + EC:DEC based half-cell. Comparatively quick metal-cation reduction and accumulation processes have been observed in NaClO 4 + PC and LiPF 6 +EC:DMC based half-cell anodes.

Electrochemical performance of nanospheric glucose derived hard carbon based electrodes, speciall... more Electrochemical performance of nanospheric glucose derived hard carbon based electrodes, specially derived from D-glucose via hydrothermal carbonization and subsequent pyrolysis at 1100◦C, has been studied in 1 M NaClO4 propylene carbonate electrolyte. High Na+
electroreduction and Na oxidation peaks were observed in cyclic voltammograms. Galvanostatic charge/discharge measurements demonstrated high specific capacity, over 300 mAh g−1 for the first cycle. After 200thcycle, a specific capacity of 160 mAh g−1(at 50 mA g−1 cycling rate) has been calculated for the nanospheric hard carbon based half-cells. Raman spectroscopy, inductively coupled plasma mass spectrometry and energy-dispersive X-ray spectroscopy data indicated accumulation (adsorption/absorption) of Na onto/into the electrochemically polarized porous nanospheric hard carbon material. Impedance data demonstrated that electrode potential has a noticeable influence on the total polarization, and charge transfer resistance as well as on the mass transfer characteristics at the carbon | (1 M NaClO4 + propylene carbonate) electrolyte interface.

The electrochemical characteristics of supercapacitors consisting of molybdenum carbide derived c... more The electrochemical characteristics of supercapacitors consisting of molybdenum carbide derived carbon C(Mo2C) electrodes in 1M NaPF 6 solutions in various mixtures (0.5–5%) of vinylene carbonate (VC) with propylene carbonate (PC) and ethyl acetate (EA) (1:1 by volume) have been studied using cyclic voltammetry, constant current charging/discharging and electrochemical impedance spectroscopy methods. The specific capacitance and series resistance values dependencies on the used solvent mixture and applied temperature (from −40 • C to 60 • C) have been established. The region of ideal polarizability has been established for C(Mo 2 C) electrodes in all electrolytes and temperatures investigated, except at T ≥ 40 • C. Specific conductivity values have been measured and correlated with electrochemistry data. Limiting capacitance, calculated characteristic time constant and complex power values depend noticeably on the solvent mixture used in the electrolyte, i.e. on the viscosity and specific conductivity of the electrolyte solution. The studied electrolytes are potential candidates for low-temperature supercapacitors.

Na-salt non-aqueous electrolyte Low-temperature supercapacitor Carbide derived carbon Solvent mix... more Na-salt non-aqueous electrolyte Low-temperature supercapacitor Carbide derived carbon Solvent mixture for wide temperature range a b s t r a c t