Article category: Full Paper

Subcategory: Lithium Ion Batteries

Dual additives for stabilizing Li deposition and SEI formation in anode-free Li metal batteries

Baolin Wu, Chunguang Chen,^* Dmitri L. Danilov,^* Zhiqiang Chen, Ming Jiang, Rüdiger-A. Eichel and Peter H.L. Notten^*

Baolin Wu, Dr. Chunguang Chen, Dr. Dmitri L. Danilov, Prof. Rüdiger-A. Eichel, Prof. Peter H.L. Notten

Forschungszentrum Jülich (IEK-9), D-52425, Jülich, Germany;

Baolin Wu, Prof. Rüdiger-A. Eichel

RWTH Aachen University, D-52074 Aachen, Germany.

Dr. Chunguang Chen

LNM, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, China;

School of Engineering Sciences, University of Chinese Academy of Sciences, Beijing 100049, China.

Dr. Dmitri L. Danilov, Dr. Zhiqiang Chen, Prof. Peter H.L. Notten

Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

Dr. Ming Jiang

Insitute of Crabon Neutrality and New Energy, School of Electronics and Information, Hangzhou Dianzi University, Hangzhou 310018, China.

Prof. Peter H.L. Notten

University of Technology Sydney, Broadway, Sydney, NSW 2007, Australia

E-mail: p.h.l.notten@tue.nl ; chenchunguang@imech.ac.cn; D.Danilov@tue.nl

Keywords : Dual-additives, Li growth, SEI formation, anode-free lithium metal battery, in situ Raman

Abstract. Anode-free Li-metal batteries (AFLBs) are of significant interest to energy storage industries due to their intrinsically high energy. However, the accumulative Li dendrites and dead Li continuously consume active Li during cycling. That results in a short lifetime and low Coulombic efficiency (CE) of AFLBs. Introducing effective electrolyte additives can improve the Li deposition homogeneity and solid-state interphase (SEI) stability for AFLBs. Herein, we reveal that introducing dual additives, composed of LiAsF₆ and FEC, into a commercial carbonate electrolyte will boost the cycle life and average CE of NCM||Cu AFLBs. The NCM||Cu AFLBs with the dual additives exhibit a capacity retention of about 75% after 50 cycles, much higher than those with bare electrolytes (35%). The average CE of the NCM||Cu AFLBs with additives can maintain 98.3% over 100 cycles. In contrast, the average CE without additives rapidly decline to 97% after only 50 cycles. In situ Raman measurements reveal that the prepared dual additives facilitate denser and smoother Li morphology during Li deposition. The dual additives significantly suppress the Li dendrite growth, enabling stable SEI formation on anode and cathode surfaces. Our results provide a broad view of developing low-cost and high-effective functional electrolytes for high-energy and long-life AFLBs.

1. Introduction

Lithium-ion batteries (LIBs) have become the critical component and technology limiting the driving range of electric vehicles (EVs).^[1] Boosting the energy density of LIBs will certainly lengthen EVs’ range. Li metal is regarded as the most attractive negative electrode for high-energy batteries due to its high theoretical specific capacity (3860 mAh·g^-1) and low redox potential (-3.04 V vs. standard hydrogen electrode).^{[2, 3]} However, Li metal anode commonly suffers from intensive side reactions, low Coulombic efficiency (CE), and rapid capacity fading. These problematic issues originate from Li dendrite growth during Li plating/stripping and the continuous formation of solid electrolyte interphase (SEI), which consumes cyclable lithium.^[4-6] Additionally, Li metal batteries often use thick Li metal foils at the anode side, which reduces batteries’ energy density and causes safety concerns.^[7]

Anode-free Li metal batteries (AFLBs) with current-collector (Cu)||electrolyte||cathode||current-collector cell configuration can be used to overcome the drawbacks of conventional Li metal battery systems.^{[8, 9]} In AFLBs, the cathode offers active Li source for being plated on the Cu or other type of current-collector substrate. In this sense, no excess of Li metal is present in pristine AFLBs.^{[10, 11]} The absence of Li metal anode leads to less weight, less volume, lower manufacturing costs, and therefore higher energy density than conventional Li metal batteries.^{[12, 13]} The anode-free design was previously considered impractical due to the short cycle life and low CE, caused by unstable SEI formation, Li dendrites, and dead Li metal.^[9] The Li source in AFLBs is only provided by the cathode material. Therefore the formation of SEI and dead Li metal rapidly consume cyclable Li-ions, drastically decreasing battery capacity.^[8] The CE of Li deposition must be considered while developing practical AFLBs to achieve high energy density and recyclability.

Electrolyte engineering effectively improves the CE by manipulating the Li deposition growth and the SEI composition.^[14]In fact, engineering electrolyte is an elaborate combination of several methods, including altering organic solvents and salt chemistry, adding additives, and manipulating concentrations of all components. Introducing a functional additive is the most promising among these electrolyte engineering methods. This method can effectively control lithium metal’s plating/exfoliation behavior and modify the SEI layer’s chemistry. Additives are applied in trace doses (usually less than 5%), making balancing the performance and cost easier.^{[15, 16]} Electrolyte additives can be categorized into two groups: inorganic additives such as LiNO₃^[17] and AlCl₃^[18], and organic additives like vinylene carbonate (VC)^[19] and fluoroethylene carbonate (FEC).^[20] The fluorine-containing electrolyte additives have superior kinetic reactivity, which facilitates the formation of more LiF components in the SEI layers to improve stability.^[21] FEC is one of Li metal batteries’ most celebrated electrolyte additives. It forms a stable yet elastic LiF-rich SEI on Li metal.^[22] Recently, many fluorine-containing salts have also been explored as additives, such as LiBF₄ and LiAsF₆.^[23] However, in contrast with Li metal batteries, a single additive is insufficient for effective application in AFLBs due to the complexity of electrochemical processes involved. Therefore, exploring suitable additives for anode-free systems is essential for developing high-performance batteries.

Herein, a facile and effective dual additive of LiAsF₆and FEC in a commercial carbonate electrolyte (1M LiPF₆-EC/DEC/DMC (1:1:1 vol%)) is reported. The proposed dual additive enhances the electrochemical performance of the AFLBs using NCM532 as the cathode and bare Cu foil as the anode current collector. The NCM||Cu cell possesses an excellent electrochemical performance with high-capacity retention of ~75% after 50 cycles and an average CE of ~98.3% in the presence of dual additives. In situ Raman measurements were performed to investigate the effect of the dual additives of LiAsF₆ and FEC on the morphology of deposited Li metal and formed SEI layers. The synergistic effect of LiAsF₆ and FEC contributes to smoother lithium plating. It suppresses the formation of Li dendrites and dead Li metal. Furthermore, F-rich SEI and CEI layers were observed to form on the Li anode and NCM-cathode surfaces, facilitating the stability of anode-free cells.