High‐Temperature Treatment of Li‐Rich Cathode Materials with Ammonia: Improved Capacity and Mean Voltage Stability during Cycling (original) (raw)

2017, Advanced Energy Materials

Since the discovery by Thackeray and Rossouw in 1991 that acid could activate Li 2 MnO 3 , [1] followed by the discovery by Gopukumar and co-workers in 1999 that this inactive phase could be activated electrochemically, [2] interest in integrated Li 2 MnO 3 and composite phases has grown rapidly. [3] The Li 2 MnO 3 phase can deliver a high theoretical capacity based on Li extraction of ≈460 mA h g −1 , though practical realities limit the total extracted capacity to ≈100 mA h g −1 , often with rapid capacity decay. [4] Attempts at stabilizing this material through lattice doping or forming "composites" with alternative materials mirrored struggles seen in the layered transition metal (TM) oxide field. For instance, LiNiO 2 suffers Ni 2+ mixing in the Li + layer that causes dramatically different capacities for different synthesis parameters, [5] and LiMnO 2 does not easily form a layered R-3m structure, [6] rather forming the orthorhombic phase, pmnm with a zigzag structure that causes low rate capability and capacity. [7] Incorporation of other Co, Mn, or Ni into pure lithiated transition metal oxides alleviated problems found with the pure, single LiTMO 2 materials. [8] Using a similar strategy, integration of Li 2 MnO 3 with layered LiMO 2 and spinel components produced electrode materials that could perform >300 mA h g −1 discharge capacity reversibly. [3,9] Subsequent patent applications were granted to Thackeray and co-workers on integrated materials containing Li 2 MnO 3 and both the layered TM oxides [10] and with spinel material. [11] Integrated xLi 2 MnO 3 •yLiNi a Co b Mn c O 2 (x + y = 1, a + b + c = 1) (Li-rich) materials are of particular interest since they have large, stable capacities above 250 mA h g −1 , though initial charging during the activation step can reach much higher, even up to >500 mA h g −1. [12,13] Activation is a complex process involving oxygen release, Li 2 O extraction, and the formation of surface spinel. [13-15] The spinel-phase domains formed can prevent to some extent further oxygen release from the bulk of the material during cycling, something which the material is susceptible to since it is believed that along with TMs, O 2− acts as a Li-rich electrode materials of the family xLi 2 MnO 3 •(1−x)LiNi a Co b Mn c O 2 (a + b + c = 1) suffer a voltage fade upon cycling that limits their utilization in commercial batteries despite their extremely high discharge capacity, ≈250 mA h g −1. Li-rich, 0.35Li 2 MnO 3 •0.65LiNi 0.35 Mn 0.45 Co 0.20 O 2 , is exposed to NH 3 at 400 °C, producing materials with improved characteristics: enhanced electrode capacity and a limited average voltage fade during 100 cycles in half cells versus Li. Three main changes caused by NH 3 treatment are established. First, a general bulk reduction of Co and Mn is observed via X-ray photoelectron spectroscopy and X-ray absorption near edge structure. Next, a structural rearrangement lowers the coordination number of CoO and MnO bonds, as well as formation of a surface spinellike structure. Additionally, Li + removal from the bulk causes the formation of surface LiOH, Li 2 CO 3 , and Li 2 O. These structural and surface changes can enhance the voltage and capacity stability of the Li-rich material electrodes after moderate NH 3 treatment times of 1-2 h.