2.5 V Battery with Stripping/Plating of Stainless Steel
Authors: Wang Huimin , Yu Denis Yau Wai 
Published: February 2020

Existing materials in lithium-ion batteries have reached their maturity, and new electrochemical systems are needed to meet the rising demand in energy storage. Here we proposed to store energy with the stripping and plating of metal ions. For example, a Fe ↔ Fe2+ + 2e-
reaction is expected to give a voltage of 2.6 V vs. Li/Li+ with a theoretical capacity of 959 mAh g-1 and an energy density of 2493 Wh kg-1, which is about 5 times more than the practical energy density of LiCoO2. In addition, the system is free from complex material synthesis processes, and can turn cheap and abundant metals into potential high-energy batteries.
To enable high voltage and prevent gas evolution during cycling, aprotic electrolyte with large stability window is needed. We demonstrate the feasibility of the system with stainless steel (410L) as the working electrode and Li metal as the counter electrode. 1M LiPF6 in EC/DEC = 1:1 by volume with 0.05M LiCl is used as the electrolyte. Cl- is added to the electrolyte to facilitate the dissolution of the metal into the electrolyte. An anion exchange membrane (Fumasep FAPQ310-PP) is used to reduce cross-over of Fe2+ to the counter electrode. Figure 1a shows the charge-discharge curves of our battery with a capacity limitation of 150 mAh g-1. A flat discharge voltage of 2.5 V vs. Li/Li+ and a stable discharge capacity are observed. Currently, the coulombic efficiency of the battery is about 87% (see Figure 1b). We have demonstrated here that the stainless-steel cathode can also be coupled with a graphite anode. During charging, Fe2+
is dissolved out from the cathode while Li+ is inserted into graphite at anode. The SS410L-graphite battery was evaluated at a constant current of 10 mA g-1 under a charge capacity limitation of 100 mAh g-1. After a few cycles of activation, a stable charge-discharge profile is established (Figure 1c). Charging voltage starts at 2 V with an initial sloping region until about 2.7 V followed by a voltage plateau at about 2.9 V. During discharge, the average cell voltage is 2.3 V. And the full cells can sustain more than 70 cycles with a CE closing to 93% (Figure 1d).
We proposed here a new inexpensive metal cathode based on stripping/plating mechanism and the feasibility was demonstrated with stainless steel. The new concept may reduce the cost of energy storage, as stainless steel is a standard material with large production capacities. In particular, as a multi-electron transfer for the redox reaction of iron, a higher theoretical capacity can be expected. Our demonstration will lead to new developments of low-cost cathode materials to meet the growing demands for energy storage devices. More results on the charge-discharge mechanism and the effect of anion exchange membranes will be presented at the meeting.
Figure 1. (a) Charge/discharge profiles and (b) cycle performance of SS410L-Li half cell with F310 AEX membrane at 10 mA g-1 with a charge capacity limitation of 150 mAh g-1. (c) Charge/discharge profiles and (d) cycle performance of SS410L-graphite battery with F310 AEX membrane at 10 mA g-1 with a charge capacity limitation of 100 mAh g-1.

Figure 1