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Page 2 of 14 Shi et al. Energy Mater 2023;3:300036 https://dx.doi.org/10.20517/energymater.2023.27

INTRODUCTION
Primary lithium-metal batteries (LMBs) provide energy densities far beyond any other electrochemical
[1]
energy storage device and have been an everyday commodity since the 1970s , with energy densities of up
to more than 700 Wh kg , which remain unmatched by any rechargeable battery. On the other hand,
-1
secondary LMBs, despite extensive efforts by electrochemists and material scientists for almost half of a
century, were commercialized only in 2004, and they represent today a niche market compared to the
dominant lithium (Li)-ion technology that uses a graphite anode instead of lithium metal. The latter,
however, is closing the gap between its theoretical and practical energy densities. Thus, the current push for
a decarbonized economy calls for a switch from graphite insertion anodes to lithium metal for a tenfold
increase in specific capacity enabling longer drive-range electric vehicles (EVs).

LMBs, however, suffer from practical limitations that prevent them from competing with Li-ion batteries.
The slow Li-ion transport at room temperature (and below) requires high operation temperatures, and the
limited anodic stability of the polyether-based solid polymer electrolytes (SPEs) used implies using low
voltage cathodes, which is detrimental to energy density. In fact, their operation temperatures could, in
principle, decrease to 40 °C, thanks to strategies to make poly(ethylene oxide) (PEO)-based SPEs
amorphous and conductive below the melting point of PEO . However, the practical operation
[2,3]
temperature of the current generation of commercial lithium-metal polymer batteries (LMPBs) is still above
70 °C, which requires either pre-heating or maintaining the temperature of the battery constantly elevated.
Even at these temperatures, though, fast charging is impossible. It is due, on the one hand, to the rapid
growth of lithium dendrites that occurs when the current density reaches a diffusion limit at which the
lithium salt concentration gradients in the electrolyte are so steep that the lithium concentration at the
lithium metal electrode reaches zero , which results in fast short-circuits. Additionally, inhomogeneous
[4]
deposition of lithium usually occurs below this limit, mainly due to the passivation layer at the lithium
surface, the so-called "Solid Electrolyte Interphase" (SEI) . The SEI might induce an inhomogeneous
[5,6]
current density, leading to protrusion of lithium metal and SEI cracking, snowballing to more
inhomogeneous deposition and possibly local depletion of lithium that then triggers dendrite growth along
local (spherical) salt concentration gradients , resulting in the formation of the so-called "mossy lithium"
[7]
over cycling, which is extremely detrimental for cell performance and safety.

As lithium is a ductile metal, lithium protrusions can be tackled by using polymer electrolytes with sufficient
mechanical stability or engineering the interface [9-14] to prevent lithium protrusion (i.e., by mechanical
[8]
confinement and ensuring homogeneous lithium transport) and the chain of events leading to mossy
lithium growth below the limiting current density. However, the fundamental issue of lithium depletion
both at high and moderate current rates is directly linked to anionic mobility and diffusion-controlled Li
+
transport. Besides the problematic dendrite growth, diffusion-controlled Li-ion transport means that the
electrolyte resistance increases during charge or discharge when the concentration gradients are steep (i.e.,
the currents are high). Thus, suppressing anion mobility logically results in a decreased internal resistance
independent of current, thereby lowering heat generation and energy losses at high currents.

Since the 1990s, single-ion conductors with fixed anions have been proposed to prevent lithium depletion
during charge [3,15-18] . Among these, rigid solid-state electrolytes, such as ceramics or glasses, impose finding
solutions to buffer not only the large anode volume changes upon cycling but also those more limited
cathode materials . On the other hand, Elastomeric polyanions, which are lithium salts where the anionic
[19]
moieties are linked to a polymer backbone, allow easier processing and buffering electrode volume changes,
similar to SPEs that use a lithium salt dissolved in a polymer matrix. The achievement of a true single-ion