Improved stability of nano-Sn electrode with high-quality nano-SEI formation for lithium ion battery

Highlight

• Nano-scaled bare Sn electrode with high-quality SEI is designed to improve stability for LiB.

• An additive removes highly oxidized carbon compounds in SEI to form high-quality SEI.

• The high-quality SEI improves greatly rate-capability and capacity of nano-bare Sn electrodes.

• The SEI can improve stability of Sn electrode for LiB without complex material treatments.

Abstract

Sn materials offer high theoretical capacities in lithium ion batteries, but they must have good cycling stability and high rate-capability in order to be commercialized. Complex and costly material treatments of Sn have been effective in reducing capacity fade, but conventionally produced bare Sn is desired for reducing cost. One simple method is to form a high-quality solid electrolyte interphase (SEI) on Sn particles with low resistance and high passivation. Fluoroethylene carbonate (FEC) added to the electrolyte forms a protective and less-resistant SEI on Sn particles during the in-situ electrochemical SEI formation cycle. FEC is a good oxidizing agent that removes highly oxidized carbon compounds and makes a SEI thinner during an oxidation process (delithiation) of SEI formation cycle. The high-quality SEI greatly improves the rate-capability and capacity of nano-sized bare Sn electrodes without any treatments: minimal capacity fade (0.014% cycle⁻¹) at 320 mA h g⁻¹ (1.3 C) for 150 cycles. The mitigating effect of FEC on capacity fade is not seen with electrodes fabricated from micro-scale (0.1~0.2 μm) Sn. The long lithium-ion diffusion path makes these micro-sized materials susceptible to decrepitation during repeated volume changes.

Introduction

Li-ion batteries (LiBs) are the predominant energy storage system for electrical vehicles (EVs) as well as various consumer electronic devices. Their key attributes are long-term performance stability, good rate capability, and thermal stability in practical use [1], [2], [3], [4], [5]. However, one of the most critical challenges of the present commercial Li-ion batteries is insufficient capacity leading to a need for frequent charging. Hence, the development of new electrode materials with higher energy density is of significant interest [4].

Tin (Sn) is a promising material for the anode because Sn is inexpensive, naturally abundant, and has a high theoretical energy density (7260 mA h cm⁻³) and specific energy [6], [7]. In contrast to the intercalation of lithium in graphite, Sn forms an alloy with lithium. Alloying and de-alloying occur reversibly according to the following equation:

xLi+xe⁻+Sn_y↔Li_xSn_y

For each Sn atom up to 4.4 Li ions are added (Li₂₂Sn₅ alloy), corresponding to a specific capacity of 992 mA h g⁻¹, which is 2.7 times higher than that of graphite (~372 mA h g⁻¹). Still, Sn and Sn based materials are difficult to commercialize due to severe capacity fading during battery cycling [7]. Similar to lithium–silicon systems, this fade is associated with the large volume expansion, up to 300%, when Sn is fully alloyed with Li [6], [7], [8]. Repeated alloying and de-alloying make Sn-based materials susceptible to pulverization, and concomitantly, the continuous formation of SEI compounds during cycling. This behavior leads to loss of cyclable lithium, consumption of electrolyte, and compromised electrical contact between particles, and hence, continuous capacity fading [7].

There have been many attempts to mitigate this capacity fade so that Sn can be used in a practical Li-ion battery; (i) Sn alloys, such as Sn–Ni [9], [10], [11], Sn–Cu [12], Sn–Co [13], Sn–Sb [14], [15], (ii) Sn–carbon material composite, such as carbon coated Sn [8], [16], [17], [18], [19], [20], Sn–CNT [21], and Sn–graphene [22], [23], [24], [25], [26], (iii) structural modification, such as nano-scaled Sn on nano-material [27], [28], [29] and hollow- [30], [31]/cube-shaped Sn [32], and (iv) combinations of (i)–(iii) [8], [27], [28], [29], [30], [31], [32]. Among them, several efforts were encouraging: high capacity, high rate-capability, and low capacity fade. However, compared to graphite, the fabrication processes of modified Sn and Sn based alloy materials are complex and expensive, representing a barrier to commercialization. Specifically, the key obstacles are poor reproducibility, low production yields, low rates of production, and safety hazards [16], [20]. Hence, to spur commercialization, electrode materials that use Sn without complex treatments while still retaining high stability and good rate-capability are needed.

Herein, we report on the formation of a robust solid electrolyte interphase (SEI) that protects against pulverization and where the SEI is formed for the most part only during the first electrochemical cycle. The process is referred to ‘in-situ electrochemical SEI formation cycle’ in this paper. A high-quality SEI is needed to maintain high performance, and it should be uniform, adhere well to the negative electrode, and have high ionic and electronic conductivities [33], [34], [35]. The SEI is formed from the reduction and polymerization of the electrolyte solvents. Additives to the electrolyte can improve the quality and composition of SEI [36], [37], [38], [39]. In particular, fluoro-ethylene carbonate (FEC) has been shown to be effective in reducing irreversible capacity loss and lowering capacity fade for several carbon and silicon based anode materials [36], [37], [38], [40], [41]. The effects of the FEC additive on SEI properties of bare Sn electrode and the formation and degradation mechanisms of the SEI on Sn anode are not yet fully understood.

In this context, we synthesized nano- and micro-sized bare Sn materials for use in a lithium ion battery and investigated their electrochemical and morphological properties with and without FEC as an electrolyte additive. Specifically, the formation and degradation mechanisms of nano-/micro-scaled Sn electrodes with different qualities SEI were scrutinized.