Ionic and electronic work functions of prototypical electrode materials, i. e. Li_xFePO₄ and Li_xMn₂O₄, in lithium ion batteries have been measured as a function of x, i. e., the state of charge. The results are discussed in the context of thermodynamic Born cycles. It is concluded that interfacial properties must be highly relevant.

Abstract

During charging of a lithium ion battery, electrons are transferred from the cathode material to the outer circuit and lithium ions are transferred into the electrolyte. Here, the energy required to take electrons and lithium ions out of two prototypical cathode materials, Li_xFePO₄ and Li_xMn₂O₄ is investigated as a function of the state of lithiation, x. Both electronic and ionic work functions vary significantly with x for Li_xFePO₄ but rather little for Li_xMn₂O₄. The relevance of these work functions for the thermodynamic description of lithium ion batteries is discussed.