Switching an Analogue to Digital Computing Property Based on Memristance By Lithiation – Opportunities on Lithium Titanates
Authors: Gonzalez-Rosillo Juan Carlos , Mullin Kaitlyn M , Pfenninger Reto , Balaish Moran , Rupp Jennifer L.M. 
Published: February 2020

Electrochemical devices allowing to modulate their resistive state through ionic motion upon electric bias are known as memristors. Currently, memristors are positioned as a major candidate to overcome the bottlenecks in current electronic-based transistors in terms of downscaling capabilities and energy consumption. The vast majority of memristor are based on two types of ions: either oxygen vacancy migration, in the so called Valence Change Memories (VCM), or a metal cation, usually Ag+ and Cu2+, in the so called Electrochemical Metallization Cells (ECM). Despite the excellent performance of both systems, a widespread implementation of oxygen-based memristors in today’s integrated circuits is delayed due to the need to address cycle-to-cycle and device-to-device variabilities while circumventing electroforming, which are inherent issues associated to the filamentary nature of the switching mechanism. Recently, Li-ion is emerging as an alternative, given the higher diffusivity of Li+ when compared to oxygen, and the ability of Li-oxides solid state conductors to accumulate and deplete lithium at the interfaces and bulk. The scarce literature regarding Li-based memristors focuses mainly in a traditional high-voltage cathode material, LixCoO2
2, in a rather slow lithium conductor as LiLaTiO3
3 and in a magnetic spinel LiFe5O8
4. Performance-wise, Li-based oxides have shown promising applications in neuromorphic computing thanks to a close-to-analog switching response. However, it is still remains unclear the defect chemistry leading to the switching behavior of Li-based materials and the impact of lithiation degree on their performance. Among the potential Li-based oxides showing resistive switching effects, Lithium titanates compounds present intrinsic properties that could be beneficial to implement a Li- based memristor: a) the existence of a metal-insulator transition upon lithium intercalation, concomitant with a phase transition and valence change in the Ti cation, b) the spinel to rock-salt phase transition occurs with a neglectable volume change and c) there is a large difference in electronic conductivity and diffusivity between the two phases. In this work, we exemplify the resistive switching capabilities of these systems for different lithium stoichiometries. To exemplify this, we will report for the first time the non-volatile, non-filamentary bipolar resistive switching characteristics of lithium titanates compounds, Li4+3xTi5O12, as a function of the lithiation degree. We have employed a recently proposed strategy to overcome lithium loss during thin film deposition and finely control the final lithiation degree of the films5 to create a stoichiometrically lithiated Li4Ti5O12 spinel phase and a highly lithiated Li7Ti5O12 rock- salt phase memristive devices. Changing the Li-content from a stoichiometrically lithiated spinel phase to a highly lithiated rock-salt phase results in the capability to tune the performance in a wide range in terms of accessible resistance window (from ratios of 102 to 106 at low voltage operation, respectively), symmetry (from highly asymmetric to symmetric behavior, respectively) and retention (from few minutes up to 105 s at room temperature, respectively), among others. In other words, controlling the lithiation degree might offer a suitable path to reduce stochasticity from which current filamentary memristive devices inherently suffer, mainly due to the difficulties to control the amount of vacancies generated, and paves the way to further control of ionic migration for novel nanoelectronic devices.

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