Negative Thermal Expansion Behavior Enabling Good Electrochemical-Energy-Storage Performance at Low Temperatures.

Metal-ion batteries (such as lithium-ion batteries) are very popular energy-storage devices nowadays. However, low temperatures cause their poor electrochemical kinetics and performance, significantly limiting their wide applications in cold environments. Here, we propose that electrochemical energy-storage materials with negative-thermal-expansion (NTE) behavior can enable good low-temperature electrochemical performance, which becomes a new strategy to tackle the low-temperature issues of metal-ion batteries. LiTi(PO) (LTP) with an a-direction thermal expansion coefficient of -1.1×10 K is used as a model material. As the temperature decreases, the transverse vibration of O atoms not only increases the transverse distances among O atoms connected to Li/Ti atoms, but also widens the Li-transport channels and enlarges the Li-insertion sites along the [12 ] direction, which are mainly controlled by the lattice parameter a. Consequently, carbon-coated LTP (C-LTP) retains good electrochemical performance at -10 °C, including fast Li diffusivity (84 % of that at 25 °C), large capacity (96 % of the theoretical capacity), and superior rate capability (83 % capacity retention at 5 C vs. 0.5 C). Moreover, the more open crystal structure of LTP at the lower temperature allows smaller maximum unit-cell-volume expansion, resulting in better cycling stability of C-LTP at -10 °C (96.8 % capacity retention over 1000 cycles at 2 C).