The Dynamical Role of Optical Phonons and Sublattice Screening in a Solid-State Ion Conductor

Solid-state electrolytes (SSEs) require ionic conductivities
that
are competitive with liquid electrolytes to realize applications in
all-solid-state batteries. Although candidate SSEs have been discovered,
the underlying mechanisms enabling superionic conduction (>1 mS
cm–1) remain elusive. In particular, the role of
ultrafast
lattice dynamics in mediating ion migration, which involves couplings
between ions, phonons, and electrons, is rarely explored experimentally
at their corresponding time scales. To investigate the complex contributions
of coupled lattice dynamics on ion migration, we modulate the charge
density occupations within the crystal framework and then measure
the time-resolved change in impedance on picosecond time scales for
a candidate SSE, Li0.5La0.5TiO3 (LLTO).
Upon perturbation, we observe enhanced ion migration at ultrafast
time scales. The respective transients match the time scales of optical
and acoustic phonon vibrations, suggesting their involvement in ion
migration. We further computationally evaluate the effect of a charge
transfer from the O 2p to the Ti 3d band on the electronic and physical
structure of LLTO. We hypothesize that the charge-transfer excitation
distorts the TiO6 polyhedra by altering the local charge
density occupancy of the hopping site at the migration pathway saddle
point, thereby causing a reduction in the migration barrier for the
Li+ hop. We rule out the contribution of photogenerated
electron carriers and laser heating. Overall, our investigation introduces
a new spectroscopic tool to probe fundamental ion hopping mechanisms
transiently at ultrafast time scales, which has previously only been
achieved in a time-averaged manner or solely via computational methods.

固态电解质（solid-state electrolytes, SSEs）需具备可与液态电解质相媲美的离子电导率，方能满足全固态电池的应用要求。尽管已发现多款候选固态电解质，但实现超离子传导（superionic conduction，>1毫西门子·厘米⁻¹）的内在机制仍不明晰。尤为关键的是，涉及离子、声子（phonons）与电子耦合作用的超快晶格动力学在调控离子迁移中所扮演的角色，仍鲜有在对应时间尺度上开展的实验探索。
为探究耦合晶格动力学对离子迁移的复杂调控作用，本研究以候选固态电解质Li₀.₅La₀.₅TiO₃（LLTO）为研究对象，通过调控其晶体骨架内的电荷密度占据（charge density occupations）情况，并在皮秒（picosecond）时间尺度上测量其阻抗（impedance）的时间分辨变化。在施加外部扰动后，我们观测到超快时间尺度下的离子迁移得到显著增强。相应的瞬态响应与光学声子（optical phonon）、声学声子（acoustic phonon）的振动时间尺度高度匹配，表明声子直接参与了离子迁移过程。
我们进一步通过计算模拟，评估了O 2p轨道向Ti 3d能带的电荷转移（charge transfer）对LLTO电子结构与物理结构的影响。我们提出如下假说：电荷转移激发通过改变迁移路径鞍点（saddle point）处跳迁位点（hopping site）的局域电荷密度占据状态，使TiO₆多面体发生畸变，进而降低了锂离子（Li⁺）跳迁的迁移势垒（migration barrier）。同时，我们排除了光生电子载流子（photogenerated electron carriers）与激光加热（laser heating）的干扰作用。
综上，本研究开发出一种可在超快时间尺度下瞬态探测基础离子跳迁机制的新型光谱表征手段（spectroscopic tool），而此前这类基础机制的探测仅能通过时间平均实验方式或仅依靠计算方法完成。