Uncovering LiH Triggered Thermal Runaway Mechanism of a High‐Energy LiNi0.5Co0.2Mn0.3O2/Graphite Pouch Cell

Abstract The continuous energy density increase of lithium ion batteries (LIBs) inevitably accompanies with the rising of safety concerns. Here, the thermal runaway characteristics of a high‐energy 5 Ah LiNi0.5Co0.2Mn0.3O2/graphite pouch cell using a thermally stable dual‐salt electrolyte are analyzed. The existence of LiH in the graphite anode side is innovatively identified in this study, and the LiH/electrolyte exothermic reactions and H2 migration from anode to cathode side are proved to contribute on triggering the thermal runaway of the pouch cell, while the phase transformation of lithiated graphite anode and the O2‐releasing from cathode are just accelerating factors for thermal runaway. In addition, heat determination during cycling at two boundary scenarios of adiabatic and isothermal environment clearly states the necessity of designing an efficient and smart battery thermal management system for avoiding heat accumulation. These findings will shed promising lights on thermal runaway route map depiction and thermal runaway prevention, as well as formulation of electrolyte for high energy safer LIBs.

(a) The photographs of ARC (BTC500, HEL) for thermal runaway study. (b) Schematic illustration of the working principle.

Figure S2
Thermal stabilities of LiPF 6 and dual-salt electrolyte and their thermal runaway curves.

Figure S3
The self-heating rate curves when 5 Ah NCM523/G pouch cells (at 0% SOC and at 100% SOC, after formation process) are tested by the heat-wait-search (HWS) mode of ARC.

Figure S4
The photographs of ARC (BTC130, HEL) for thermal runaway study of battery materials.

Figure S5
XRD patterns of powders collected after thermal runaway of NCM523/G pouch cell ( 0 and 100% SOC).

Figure S6
Heat flow of pristine anode/electrolyte, and cycled anode (0% SOC) /electrolyte when tested in a STA system.

Figure S8
Schematic illustration of a self-made system containing two bomb chambers, for crosstalk effect study in ARC (BTC130, HEL).

Figure S9
The determined components percentage in collected gas after ARC testing, in a self-made two bomb chamber system containing cathode/electrolyte and anode/electrolyte separately (as illustrated in Figure S8) The calculated binding energy of H 2 , CH 4 , and CO 2 with NCM cathode

Computational methods
Periodic planewave DFT+U calculations for the cathode interface systems were performed using the Vienna ab initio Simulation (VASP) [1][2][3] within MedeA ® computational environment [4] with the spin-polarized Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional and the projectoraugmented wave (PAW) scheme to treat core electrons [5][6][7]. VASP standard pseudopotentials (PP) were used for carbon, oxygen, and hydrogen atoms. PAW PPs employed for the transition metal atoms Co, Ni and Mn (denoted as Co_pv, Ni_pv and Mn_pv, respectively) treat p semi-core states as valence states. The Li_sv PP used for Li treats the 1s shell as valence states. For the +U augmented treatment of Co, Ni and Mn 3d orbitals, we chose a U eff value of 4.91eV for Co, 5.96 eV for Ni and 5.0 eV for Mn [8,9]. To describe the weak van der Waals'force (VDW), the DFT-D3-BJ-damping correction was adopted for geometric optimization [10]. We adopted the NCM disordered configuration (Monoclinic phase) with distributed Co, Ni and Mn ions. The lattice parameters, cell volumes and the atomic positions are fully optimized and relaxed until the energy is less than 1 × 10 −5 eV per unit cell and the force on each atom is less than 0.02 eV Å −1 . We used the conjugate gradient minimization method for geometry optimization. For our simulated structures, the kinetic energy cutoffs were set at 500 eV for the plane wave basis set and the calculation were spin-polarized. A Monkhorst-Pack grid with 2×2×2 meshes was employed in the irreducible Brillouin zone [11]. The (001) surface for the fully delithiated