(Battery Division Technology Award) Insight of Structures and Properties of Cathode Materials for Li-ion Battery

Abstract

Insight into relationship between Crystal/Interface structure and properties of capacity, stability and rate capability are important for developing advanced Li-ion batteries. Using theoretical calculations combined with experimental in-situ tests, we did extensive studies on the kinetic of Li-ion diffusion for two representative cathode materials: layered Li(NixMnyCoz)O2 (NMC) (x + y + z = 1) and LiFePO4. We not only focus on the bulk kinetics, but also the kinetics across electrode/electrolyte solid-liquid interface and in the electrolytes. For example, we first proposed that "Janus" solid-liquid interface would facilitate the Li-ion transport in battery and introducing some disordering in non-active cathode materials would activate them for Li-ion storage.（Ref. 1） For high energy and power density applications (e.g., EVs), the safety becomes especially important. Using ab initio calculations combined with experiments, we clarified how the thermal stability of NMC materials can be tuned by the most unstable oxygen, which is determined by the local coordination structure unit (LCSU) of oxygen (TM(Ni, Mn, Co)3-O-Li3-x’): each O atom bonds with three of transition metal (TM) from the TM-layer and three to zero of Li from fully discharged to charged states from the Li-layer. Under this model, how the lithium content, valence states of Ni, contents of Ni, Mn, and Co, and Ni/Li disorder to tune the thermal stability of NMC materials by affecting the sites, content, and the release temperature of the most unstable oxygen is proposed. In NMC, insight of Ni/Li disorder has be investigated by theory and experimental e.g. neutron powder diffraction experiments and magnetization measurements. The spins of TM ions construct a two-dimensional triangular networks, which can be considered as a simple case of geometrical frustration. Remarkably, the frustration parameters of these compounds are estimated to be larger than 30, indicating the existence of strongly frustrated magnetic interactions between spins of TM ions, which give rise to lattice instability, the formation of Li/Ni exchange in NMC will help to partially relieve the degeneracy of the frustrated magnetic lattice by forming a stable antiferromagnetic state in hexagonal sublattice with nonmagnetic ions located in centers of the hexagons. Moreover, Li/Ni exchange will introduce 180° superexchange interaction, which further relieves the magnetic frustration through bringing in new exchange paths. Thus, the variation of Li/Ni exchange ratio vs. TM mole fraction in NMC with different compositions can be well understood and predicted in terms of magnetic frustration and superexchange interactions. (Ref. 2) Ref. (a)F. Pan el al., “Kinetics Tuning of Li-ion Diffusion in Layered Li(NixMnyCoz)O2”, J. Am. Chem. Soc., 2015, 137, 8364; (b) "Optimized Temperature Effect of Li-Ion Diffusion with Layer Distance in Li(NixMnyCoz)O2 Cathode Materials for High Performance Li-Ion Battery", Adv. Energy Mater., 2015, 1501309(1-9).(c) "Janus Solid–Liquid Interface Enabling Ultrahigh Charging and Discharging Rate for Advanced Lithium-Ion Batteries", Nano Lett,2015, 15 (9), pp 6102 (d)“Single-particle performances and properties of LiFePO4 nanocrystals for Li-ion batteries” Energy Mater., (Front page ) 2016, 1601894 (e) Nano Lett, 2017, 17, 6018 (f) Nano Energy 2017 37, 90 (g) Nano Lett., 2017, 17 (8), 4934(a) F. Pan el al., “Tuning of Thermal Stability in Layered Li(NixMnyCoz)O2”, J. Am. Chem. Soc., 2016, 138 (40), 13326, (b)“Aligned Li+ Tunnels in Core−Shell Li(NixMnyCoz)O2@LiFePO4 Enhances Its High Voltage Cycling Stability as Li-ion Battery Cathode” Nano Letters 2016, 16 (10), pp 6357; (c) Nano Letters, 2015, 15, 5590; （d）The Role of Super-Exchange Interaction on Tuning of Ni/Li Disordering in Layered Li (NixMnyCoz) O2, J. Phys. Chem. Lett., 2017, 8 (22), 5537; (e) Insight into the origin of Lithium/Nickel ions exchange in layered Li(NixMnyCoz)O2 cathode materials, Nano Energy 49 2018,49,77