Electrochemical verification of the redox mechanism of FeS₂ in a rechargeable lithium battery

Highlights

• Redox mechanism of FeS₂ is studied by galvanostatic cycling and impedance analysis.

• FeS₂ cannot be re-constituted by recharge once being fully discharged.

• Li/FeS₂ battery converts into a hybrid of a Li/FeS cell and a Li/S cell in the first discharge.

• Li/FeS₂ battery faces the same problems with Li/S battery in relation to the dissolution of polysulfide.

Abstract

The redox mechanism of micro-sized FeS₂ particles in a rechargeable lithium battery is studied by galvanostatic cycling and ac-impedance analysis. It is shown that FeS₂ is irreversibly reduced on the first discharge, turning Li/FeS₂ cell into a combination of Li/FeS and Li/S chemistries, as suggested by two distinct discharge plateaus at ∼1.5 and 2.0 V, respectively. The first discharge consists of an irreversible conversion of FeS₂ to Li₂FeS₂ intermediate and its subsequent reduction to metallic Fe and Li₂S. The first discharge suffers an initial voltage delay, suggesting that the discharge progresses in a thermodynamic non-equilibrium condition. The initial voltage delay can be attributed to the large grain boundary resistance (GBR) of pristine FeS₂ particles, which impedes the nucleation of a new solid Li₂FeS₂ phase causing high polarization. Ac-impedance spectra of the FeS₂ cathode are composed of a semicircle and a straight sloping line, representative of an electrode reaction resistance (R_er) and a Li⁺ adsorption impedance, respectively. The R_er is found to decrease progressively during the first discharge and reaches a plateau when the cell is charged above 2.5 V vs. Li/Li⁺, being consistent with the model that FeS₂ is irreversibly reduced during the first discharge and that the Li₂S/Li₂S_n redox couple is formed in recharge. It is indicated that Li/FeS₂ batteries face the same problems as Li/S batteries, such as the dissolution of lithium polysulfide, the formation of a redox shuttle, and the loss of sulfur active material.

Introduction

Iron pyrite (FeS₂) is an earth-abundant semi-conducting natural mineral that is of great interest in applications of renewable energy conversion and electrochemical energy storage because of its unique band gap (E_g = 0.95 eV) useful for solar cells [1], [2] and high capacity for Li⁺ ion storage [3], [4]. For energy storage in lithium batteries, FeS₂ has a theoretical specific capacity of 894 Ah/kg based on the complete conversion of FeS₂ into metallic Fe and Li₂S. The high specific capacity together with other merits, such as nontoxicity and cost effectiveness, make FeS₂ appealing and competitive in developing the next generation of batteries beyond Li-ion. In particular, the Li/FeS₂ primary battery operating at ambient temperature was already commercialized as early as the 1980’s [5], and FeS₂ was proven to be highly reversible in rechargeable lithium batteries operated at high temperatures (375–500 °C) [6], [7]. Other publications have shown that Li/FeS₂ batteries are capable of being cycled up to 500 cycles in a moderate temperature range of 90–130 °C by employing a composite polymer electrolyte [8], [9], [10]. However, the attempts to develop a rechargeable lithium battery at ambient temperature have not been successful due to the poor recyclability of FeS₂, which was attributed to the different redox mechanism of FeS₂ in non-aqueous electrolytes.

To date, the redox mechanism of FeS₂ in non-aqueous electrolytes has been poorly understood and is the subject of controversy in a number of publications [3], [9], [11], [12], [13], [14]. By analyzing a number of previously reported results and observations, in a recent review we proposed that FeS₂ in rechargeable lithium batteries was subject to the following processes [15]: On the initial discharge, FeS₂ is first converted (reduced) to Li₂FeS₂ during which the Fe²⁺ valance remains unchanged whilst the (SS)²⁻ bond is broken to form 2S²⁻ anions (Eq. (1)). The resultant Li₂Fe₂S₂ is then reduced to metallic Fe and Li₂S, as described by Eq. (2), which leads to an overall four-electron reduction (Eq. (3)).

FeS₂ + 2Li⁺ + 2e → Li₂FeS₂

Li₂FeS₂ + 2Li⁺ + 2e → Fe + 2Li₂S

FeS₂ + 4Li⁺ + 4e → Fe + 2Li₂S

Once being fully discharged, the original FeS₂ cannot be re-constituted by subsequent recharge, instead, two new reversible electrochemical reactions are emerged in the subsequent recharge and cycling, as shown below.

Fe + Li₂S – 2e ↔ FeS + 2Li⁺

nLi₂S – (2n − 2)e ↔ Li₂S_n + (2n − 2)Li⁺

We speculated that the formed FeS and Li₂S_n are combined into a complex (FeS⋯S_nLi₂) through the chemical interaction of sulfur atoms. In addition, the surface of FeS particles may be partially oxidized to FeS₂ by Li₂S_n, as described by Eq. (6).

FeS + Li₂S_n → FeS₂ + Li₂S_n−1

However, the above are only a hypothesis, and there is a need to support our hypothesis using experimental results.

In our previous work on Li-ion batteries, we successfully identified the essential potential regions for the formation of a robust solid electrolyte interface (SEI) by correlating the ac-impedance results with the cell’s potential for graphite anode [16], [17], spinel LiMn₂O₄ [18] and a layered nickel-based cathode [19]. We observed that the formation and growth of an SEI as well as the change of the electrode reaction kinetics were well characterized by the electrochemical impedance spectroscopy (EIS) of a cell. It is believed that the simple EIS technique is also suitable for the analysis of the redox mechanism of FeS₂ cathode in rechargeable Li/FeS₂ batteries. In the present work, we study and discuss the redox mechanism of FeS₂ cathode material by analyzing the voltage profile and impedance spectrum of Li/FeS₂ cells.