Investigating the Decomposition of Delithiated LiNi1-XMxO2 (M = Al, Co, Mn or Mg, x = 0 or 0.05)

Abstract

As lithium ion battery technology expands into more demanding applications such as electric vehicles, attention has shifted towards nickel-rich positive electrode materials, namely LiNi1-x-yMnxCoyO2 (NMC) and LiNi1-x-yCoxAlyO2 (NCA).1 Aims to improve energy density and reduce costs of NMC and NCA can be achieved by increasing the Ni content of the material, and the compositions of the two materials will invariably converge towards LiNiO2 (LNO). However, there are several issues that LNO and Ni-rich materials experience, chief among them being the thermal instability of the material when delithiated.1 The safety of Li ion batteries is a big concern for manufacturers and consumers. When a short circuit occurs, a large current flows through the short, generating a large amount of heat in a short time. If the cell cannot dissipate the generated heat quickly enough, the cell overheats. As the internal temperature of the cell rises, it triggers several reactions.2 On the negative electrode side, the metastable components of the negative electrode solid electrolyte interphase (SEI) decompose and the intercalated lithium at the negative electrode reacts with the electrolyte. On the positive electrode side, the delithiated positive electrode material decomposes when heated sufficiently. This exothermic reaction releases oxygen, which reacts further with the electrolyte to release more heat, causing more decomposition. This positive feedback loop is known as a thermal runaway, and the temperature and pressure rises uncontrollably. Thermal runaway is the major contributor to Li-ion battery safety incidents, as the rapidly rising temperature and pressure may result in cells catching fire and/or exploding. Understanding the decomposition of delithiated positive electrodes may help reduce the likelihood of thermal runaway. Increasing the decomposition onset temperature may allow reactions at earlier temperatures to burn out before triggering the next set of chain reactions. Additionally, the chain may be broken if the decomposition reactions occur slowly enough that the heat dissipation can offset heat generation. As such, the thermal instability of a candidate positive electrode material is a factor when considering its suitability. Multiple studies have shown that delithiated LNO and Ni-rich derivatives are more thermally unstable than other materials such as LiCoO2 (LCO), LiFePO4 (LFP) and LiMn2O4 (LMO).2–4 The state of charge, or degree of delithiation, also factors into the decomposition temperature.3 Recent work studied the thermal instability of LNO and Al, Mg, Mn or Co doped derivatives.4 It was found that LNO with 5% Co did not reduce the reactivity of the delithiated material with the electrolyte. Conversely, LNO with 5% Al, Mg or Mn all had reduced reactivity, with the Al and Mg doped materials reducing the reactivity the most. It is not certain why some dopants reduce the reactivity of the material with electrolyte, or why 5% dopant concentrations can affect the thermal behavior. In this work, LiNi1-xMxO2 (M = Al, Mg, Mn or Co, x = 0 or 0.05) electrode materials were delithiated and heated to study the decompositions of the materials. Thermogravimetric analysis (TGA) was used to track the mass loss of the decomposition as shown in Figure 1, showing that Al and Mg doped materials have increased thermal stability. X-ray diffraction (XRD) was used to characterize materials decomposed at various temperatures. Delithiated materials were decomposed at various heating rates to study decomposition kinetics. Electrode materials were also delithiated to a lesser degree and decomposed to study the effect of Li content. (1) Xu, J.; Lin, F.; Doeff, M. M.; Tong, W. J. Mater. Chem. A 2017, 5, 874–901. (2) Liu, K.; Liu, Y.; Lin, D.; Pei, A.; Cui, Y. Sci. Adv. 2018, 4, eaas9820(1-11). (3) Dahn, J. R.; Fuller, E. W.; Obrovac, M.; von Sacken, U. Solid State Ionics 1994, 69, 265–270. (4) Li, H.; Cormier, M.; Zhang, N.; Inglis, J.; Li, J.; Dahn, J. R. J. Electrochem. Soc. 2019, 166, A429–A439. Figure 1