To meet increasing demands for energy storage, many recent battery researches are devoted to new electrode chemistries and reaction mechanisms that promise substantial increases in energy density.¹ In this paper, we intend to break away from the preconception that lithium ions have to be involved in the cathode side of a lithium-ion battery for charge transfer, but instead utilize the anion in the electrolyte for the reaction, such as that in a dual-ion battery. In particular, we are interested in exploring the possibility of using chloride ions for battery applications. To demonstrate the feasibility of the new battery system, we have chosen to use silver as our active material, as Ag/AgCl is a well-known redox reaction. The electrochemical reaction at the electrode is given by

Ag + Cl^- « Ag⁺ Cl^-(s) + e^-

The theoretical capacity of the reaction is 248 mAh g^-1. Since Ag/AgCl has a potential of 0.197 V vs. standard hydrogen electrode in aqueous solution, if a non-aqueous electrolyte with Li metal anode can be used, the expected cell voltage will be around 3 V.

In our preliminary experiments, we found that pouch cells assembled with two types of Ag powders as active materials can be charged and discharged in the electrolyte of 0.05M LiCl 1M LiPF₆ EC/DEC=1:1 with Li metal as the counter electrode. The charge-discharge performances are presented in Figure 1a, showing that the materials can give capacities of about 10 mAh g^-1 and 70 mAh g^-1 for Ag micro powder (with particle size about 0.5-4 um) and Ag nano powder (with particle size less than 150 nm), respectively with a potential of around 2.8 V vs. Li/Li⁺. To verify that the reaction is from chloride reaction, a control experiment using electrolyte without LiCl was conducted. The cell gives a plateau at about 3.7 V vs. Li/Li⁺ (probably due to reaction with PF₆^-) without the plateau at 2.8 V (Figure 1a). We also verified that the plateau is not from alloying of Ag with Li, by showing that there is no capacity if the cell is initially discharged to 2 V. In this system, the mechanism differs from the classical Li⁺ insertion/deinsertion processes. XRD study shows the formation and decomposition of AgCl during charge and discharge (Figure 1b), accompanying the absorption and release of Cl^- in the electrolyte. The results demonstrate that chloride-ion can participate in electrochemical reactions in an organic electrolyte.

We found that the Ag/AgCl electrode shows excellent discharge rate capability (removal of Cl from the material) (see in Figure 1c). When the discharge rate is increased from 5 mA g^-1 to 100 mA g^-1 (a 20-fold increase in current), a capacity of ~62 mAh g^-1 is still obtained (capacity retention of 91%). This demonstrates that the Ag/AgCl reaction has extremely fast kinetics. The electrode also shows good cycle stability with little capacity decay after 15 cycles under a current density of 10 mA g^-1 (see in Figure 1d).

Our findings demonstrate a novel battery system with chloride ions as charge carriers to assist the oxidation and reduction of the Ag on the cathode side, storing energy by the formation of AgCl. So far, the obtained capacity is only about 70 mAh g^-1, much smaller than the theoretical capacity. This is attributed to the big particles of Ag powder and insulating property of the resulting AgCl. Further work is progress to synthesize nanoparticles and carbon nanocomposites to increase the surface area and electronic conductivity. More results will be presented at the meeting.

Reference:

[1] H.-C. Yu, C. Ling, J. Bhattacharya, J. C. Thomas, K. Thornton and A. Van der Ven, Energy Environ. Sci., 2014, 7,1760.

Figure 1. Preliminary results on Ag materials with LiCl-containing electrolyte. (a) Charge-discharge curves of micro-Ag and nano-Ag. (b) XRD patterns of electrodes at different states of charge. (c) Rate performance of nano-Ag. (d) Cycle performance of nano-Ag with a current density of 10 mA g^-1.