Chemo-mechanical failure mechanisms of the silicon anode in solid-state batteries.

Silicon is a promising anode material due to its high theoretical specific capacity, low lithiation potential and low lithium dendrite risk. Yet, the electrochemical performance of silicon anodes in solid-state batteries is still poor (for example, low actual specific capacity and fast capacity decay), hindering practical applications. Here the chemo-mechanical failure mechanisms of composite Si/Li6PS5Cl and solid-electrolyte-free silicon anodes are revealed by combining structural and chemical characterizations with theoretical simulations. The growth of the solid electrolyte interphase at the Si|Li6PS5Cl interface causes severe resistance increase in composite anodes, explaining their fast capacity decay. Solid-electrolyte-free silicon anodes show sufficient ionic and electronic conductivities, enabling a high specific capacity. However, microscale void formation during delithiation causes larger mechanical stress at the two-dimensional interfaces of these anodes than in composite anodes. Understanding these chemo-mechanical failure mechanisms of different anode architectures and the role of interphase formation helps to provide guidelines for the design of improved electrode materials.

Solid-state batteries: from 'all-solid' to 'almost-solid'.

The 'all-solid' concept is not necessarily the most rewarding target, and 'almost-solid' may rather be the most feasible strategy.

Interface Design Enabling Stable Polymer/Thiophosphate Electrolyte Separators for Dendrite-Free Lithium Metal Batteries.

Organic/inorganic interfaces greatly affect Li+ transport in composite solid electrolytes (SEs), while SE/electrode interfacial stability plays a critical role in the cycling performance of solid-state batteries (SSBs). However, incomplete understanding of interfacial (in)stability hinders the practical application of composite SEs in SSBs. Herein, chemical degradation between Li6 PS5 Cl (LPSCl) and poly(ethylene glycol) (PEG) is revealed. The high polarity of PEG changes the electronic state and structural bonding of the PS4 3- tetrahedra, thus triggering a series of side reactions. A substituted terminal group of PEG not only stabilizes the inner interfaces but also extends the electrochemical window of the composite SE. Moreover, a LiF-rich layer can effectively prevent side reactions at the Li/SE interface. The results provide insights into the chemical stability of polymer/sulfide composites and demonstrate an interface design to achieve dendrite-free lithium metal batteries.

A flexible electron-blocking interfacial shield for dendrite-free solid lithium metal batteries.

Solid-state batteries (SSBs) are considered to be the next-generation lithium-ion battery technology due to their enhanced energy density and safety. However, the high electronic conductivity of solid-state electrolytes (SSEs) leads to Li dendrite nucleation and proliferation. Uneven electric-field distribution resulting from poor interfacial contact can further promote dendritic deposition and lead to rapid short circuiting of SSBs. Herein, we propose a flexible electron-blocking interfacial shield (EBS) to protect garnet electrolytes from the electronic degradation. The EBS formed by an in-situ substitution reaction can not only increase lithiophilicity but also stabilize the Li volume change, maintaining the integrity of the interface during repeated cycling. Density functional theory calculations show a high electron-tunneling energy barrier from Li metal to the EBS, indicating an excellent capacity for electron-blocking. EBS protected cells exhibit an improved critical current density of 1.2 mA cm-2 and stable cycling for over 400 h at 1 mA cm-2 (1 mAh cm-2) at room temperature. These results demonstrate an effective strategy for the suppression of Li dendrites and present fresh insight into the rational design of the SSE and Li metal interface.

Easily Decomposed Discharge Products Induced by Cathode Construction for Highly Energy-Efficient Lithium-Oxygen Batteries.

The lithium-oxygen (Li-O2) battery is deemed as a promising candidate for the next-generation of energy storage system due to its ultrahigh theoretical energy density. However, low energy efficiency and inferior cycle stability induced by the sluggish kinetics of charge transfer in discharge products limit its further development in practical application. In this work, tin dioxide (SnO2) nanoparticles decorated carbon nanotubes (SnO2/CNTs) have been constructed as composite cathodes to manipulate the morphology and component of discharge products in Li-O2 batteries. Owing to the strong oxygen adsorption of SnO2, oxygen-reduction reactions tend to occur on composite cathode surfaces, resulting in the formation of flake-like discharge products of Li2- xO2 less than 10 nm in thickness rather than toroidal particles of several hundred nanometers. Such homogeneous nanosized discharge products with lithium vacancies markedly enhance the electrode kinetics and charge transfer in discharge products. Consequently, the Li-O2 batteries based on the SnO2/CNT cathodes show a small polarization voltage gap, which leads to superior energy efficiency (80%) compared with that based on pristine CNT cathodes. The results demonstrate that optimizing the discharge products with nanosized morphology and defective component by cathode construction is an effective strategy to realize the Li-O2 batteries with increased energy efficiency and improved cycle stability.

Formation of Nanosized Defective Lithium Peroxides through Si-Coated Carbon Nanotube Cathodes for High Energy Efficiency Li-O2 Batteries.

The formation and decomposition of lithium peroxides (Li2O2) during cycling is the key process for the reversible operation of lithium-oxygen batteries. The manipulation of such products from the large toroidal particles about hundreds of nanometers to the ones in the scale of tens of nanometers can improve the energy efficiency and the cycle life of the batteries. In this work, we carry out an in situ morphology tuning of Li2O2 by virtue of the surface properties of the n-type Si-modified aligned carbon nanotube (CNT) cathodes. With the introduction of an n-type Si coating layer on the CNT surface, the morphology of Li2O2 formed by discharge changes from large toroidal particles (∼300 nm) deposited on the pristine CNT cathodes to nanoparticles (10-20 nm) with poor crystallinity and plenty of lithium vacancies. Beneficial from such changes, the charge overpotential dramatically decreases to 0.55 V, with the charge plateau lying at 3.5 V even in the case of a high discharge capacity (3450 mA h g-1) being delivered, resulting in the high electrical energy efficiency approaching 80%. Such an improvement is attributed to the fact that the introduction of the n-type Si coating layer changes the surface properties of CNTs and guides the formation of nanosized amorphous-like lithium peroxides with plenty of defects. These results demonstrate that the cathode surface properties play an important role in the formation of products formed during the cycle, providing inspiration to design superior cathodes for the Li-O2 cells.