State-of-the-art lithium (Li)-ion batteries are approaching their specific energy limits yet are challenged by the ever-increasing demand of today’s energy storage and power applications, especially for electric vehicles. Li metal is considered an ultimate anode material for future high-energy rechargeable batteries when combined with existing or emerging high-capacity cathode materials. However, much current research focuses on the battery materials level, and there have been very few accounts of cell design principles. Here we discuss crucial conditions needed to achieve a specific energy higher than 350 Wh kg−1, up to 500 Wh kg−1, for rechargeable Li metal batteries using high-nickel-content lithium nickel manganese cobalt oxides as cathode materials. We also provide an analysis of key factors such as cathode loading, electrolyte amount and Li foil thickness that impact the cell-level cycle life. Furthermore, we identify several important strategies to reduce electrolyte-Li reaction, protect Li surfaces and stabilize anode architectures for long-cycling high-specific-energy cells. Jun Liu and Battery500 Consortium colleagues contemplate the way forward towards high-energy and long-cycling practical batteries.

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Pathways for practical high-energy long-cycling lithium metal batteries

Lithium metal anodes have attracted much attention as candidates for high-energy batteries, but there have been few reports of long cycling behaviour, and the degradation mechanism of realistic high-energy Li metal cells remains unclear. Here, we develop a prototypical 300 Wh kg−1 (1.0 Ah) pouch cell by integrating a Li metal anode, a LiNi0.6Mn0.2Co0.2O2 cathode and a compatible electrolyte. Under small uniform external pressure, the cell undergoes 200 cycles with 86% capacity retention and 83% energy retention. In the initial 50 cycles, flat Li foil converts into large Li particles that are entangled in the solid-electrolyte interphase, which leads to rapid volume expansion of the anode (cell thickening of 48%). As cycling continues, the external pressure helps the Li anode maintain good contact between the Li particles, which ensures a conducting percolation pathway for both ions and electrons, and thus the electrochemical reactions continue to occur. Accordingly, the solid Li particles evolve into a porous structure, which manifests in substantially reduced cell swelling by 19% in the subsequent 150 cycles. Much has been said about the high-energy, long-lasting potential of Li metal batteries, and yet little has been demonstrated at the cell scale. Here, Jun Liu and colleagues demonstrate a Li metal pouch cell with a 300 Wh kg−1 energy density and a 200-cycle lifetime.

https://www.nature.com/articles/s41560-019-0390-6.pdf?proof=t

High-energy lithium metal pouch cells with limited anode swelling and long stable cycles

Coulombic efficiency (CE) has been widely used in battery research as a quantifiable indicator for the reversibility of batteries While CE helps to predict the lifespan of a lithium-ion battery, the prediction is not necessarily accurate in a rechargeable lithium metal battery Here, we discuss the fundamental definition of CE and unravel its true meaning in lithium-ion batteries and a few representative configurations of lithium metal batteries Through examining the similarities and differences of CE in lithium-ion batteries and lithium metal batteries, we establish a CE measuring protocol with the aim of developing high-energy long-lasting practical lithium metal batteries The understanding of CE and the CE protocol are broadly applicable in other rechargeable metal batteries including Zn, Mg and Na batteries Coulombic efficiency (CE) has been frequently used to assess the cyclability of newly developed materials for lithium metal batteries The authors argue that caution must be exercised during the assessment of CE, and propose a CE testing protocol for the development of lithium metal batteries

Understanding and applying coulombic efficiency in lithium metal batteries

Despite considerable efforts to stabilize lithium metal anode structures and prevent dendrite formation, achieving long cycling life in high-energy batteries under realistic conditions remains extremely difficult due to a combination of complex failure modes that involve accelerated anode degradation and the depletion of electrolyte and lithium metal. Here we report a self-smoothing lithium–carbon anode structure based on mesoporous carbon nanofibres, which, coupled with a lithium nickel–manganese–cobalt oxide cathode with a high nickel content, can lead to a cell-level energy density of 350–380 Wh kg−1 (counting all the active and inactive components) and a stable cycling life up to 200 cycles. These performances are achieved under the realistic conditions required for practical high-energy rechargeable lithium metal batteries: cathode loading ≥4.0 mAh cm−2, negative to positive electrode capacity ratio ≤2 and electrolyte weight to cathode capacity ratio ≤3 g Ah−1. The high stability of our anode is due to the amine functionalization and the mesoporous carbon structures that favour smooth lithium deposition. Metallic lithium wets a functionalized mesoporous carbon film to create a self-smoothing anode that, in conjunction with a standard lithium nickel–manganese–cobalt cathode, delivers long cycling life, 350 Wh kg−1 high-energy cells under realistic conditions.

Self-smoothing anode for achieving high-energy lithium metal batteries under realistic conditions

Lithium metal anodes are potentially key for next-generation energy-dense batteries because of the extremely high capacity and the ultralow redox potential. However, notorious safety concerns of Li metal in liquid electrolytes have significantly retarded its commercialization: on one hand, lithium metal morphological instabilities (LMI) can cause cell shorting and even explosion; on the other hand, breaking of the grown Li arms induces the so-called "dead Li"; furthermore, the continuous consumption of the liquid electrolyte and cycleable lithium also shortens cell life. The research community has been seeking new strategies to protect Li metal anodes and significant progress has been made in the last decade. Here, an overview of the fundamental understandings of solid electrolyte interphase (SEI) formation, conceptual models, and advanced real-time characterizations of LMI are presented. Instructed by the conceptual models, strategies including increasing the donatable fluorine concentration (DFC) in liquid to enrich LiF component in SEI, increasing salt concentration (ionic strength) and sacrificial electrolyte additives, building artificial SEI to boost self-healing of natural SEI, and 3D electrode frameworks to reduce current density and delay Sand's extinction are summarized. Practical challenges in competing with graphite and silicon anodes are outlined.

Developing High-Performance Lithium Metal Anode in Liquid Electrolytes: Challenges and Progress.

The interplay between solid electrolyte interface (SEI) and dendritic lithium growth

The solid electrolyte interphase (SEI) layer plays an important role in altering the ion transport and modifying the structural evolution of the Li metal anode during repeated cycling. While the fu...

Tyler Taverne

Papers

The interplay between solid electrolyte interface (SEI) and dendritic lithium growth

Tuning Solid Electrolyte Interphase Layer Properties through the Integration of Conversion Reaction.