Two-dimensional lithium diffusion behavior and probable hybrid phase transformation kinetics in olivine lithium iron phosphate

Lithium iron phosphate is an important lithium electrode material.
Ion batteries and a model system for studying the electrochemical drive phase transition.
Despite extensive research, many aspects of phase transition and lithium transmission in this material remain unclear.
Here we combine opera hard X-
X-ray imaging and phase
Field model to clarify single-
Long-crystal lithium iron phosphate Micron RodAlong the axis010]direction.
It is found that the diffusion coefficient of lithium is 2-
Contains ~ Size in micron particles of 3% lithiumiron anti-site defects.
Our study provides direct evidence for the previously predicted surface response. limited phase-
Potential operation of boundary migration mechanism and mixed phase growth mode, where phase-
Boundary movement is controlled by surface reaction or lithium diffusion in different crystal directions.
These findings reveal the rich stages
Under normal circumstances, the conversion behavior of lithium iron phosphate and plug-in compounds helps guide better electrode design.
With the progress of energy storage technology, many important lithium batteries have been developed.
First, the electrode material of Ion battery driven by electrolysis
The orderly phase transition of graphite anode and olive lithium iron phosphate (LiFePO)cathode.
Recent theoretical and experimental studies have revealed that (dis)charge rates.
Still, two people
In relatively low cases, phase coexistence will still dominate (dis)
Charging rate and larger Micron-
Particle size, where phase-
Has played an important role in the movement of the world (dis)Charge Dynamics.
However, in LiFePO, the morphology and migration behavior of the phase boundary (LFP)
Although many experiments and modeling efforts seek clarification, this is still a controversial topic. When first-
The order conversion consists of two phases with different components, and the growth of the new phase is often classified as limited volume diffusion (BDL)or interface-source limited.
Rate of phase-
Boundary migration is controlled by the speed at which species are transported to the boundary over a long period of time
Range Diffusion in BDL growth, or rate atoms passing through boundaries in the interfacesource-limited growth. For ion-
However, the fact that inserting material is an open system (i. e.
Exchange quality with environment)
It is possible to produce a completely new transformation dynamics in which the phase-
Boundary movement is controlled by the speed at which ions are inserted or extracted on the electrode/electrolyte interface.
This surface reactionlimited (SRL)phase-
The boundary migration mechanism is first composed of Singh, Ceder and Bazant (SCB)to explain (100)-
The orientation phase boundary observed in the partially dehydrogen LFP particles.
They believe that Li diffusion is very easy in LFP, so the surface reaction should be rate-
Limiting steps for stage growth.
In the predicted SRL dynamics, the phase boundary moves vertically to the direction of the Li surface flux at a constant speed.
However, although the SCB prediction is theoretically attractive and supported by indirect evidence, direct experimental confirmation of the SRL phase
So far, no boundary movement of any inserted compounds has been reported.
Although the field observation boundary in the previous stage is vertical (010)
The particle surface in the partially dehydrogen LFP platelet particles is considered evidence of the migration of the SRL boundary, and it is recognized that this form may be in phase-
Separation process under the action of elastic strain energy.
On the contrary, the micro LFP particles that can detect damage are currently observed in the field (de)
Lithium seems to support only the BDL phase --
Growth mechanism.
In the actual electro-chemical conditions, whether the phase transition can be operated in the SRL mode is not only a scientific and important problem, but also to improve the ion-
Insert battery material.
The conversion between the FePO and the LFP phase is subject to Li-
Diffusion behavior in LFP.
Since the first time
The principle calculation of Morgan et al.
According to the report, the diffusion of Li in the olive structure is mainly limited [010]-
Open Channel, onedimensional (1D)
Li diffusion coefficient is widely regarded as a typical feature of LFP.
However, Amin et al.
A remarkable discovery has been made that Lee has the same mobility on the millimeter axis
LFP single crystal with size of 2. 5-3% Li-Fe defense
Field defects are measured by impedance.
Later, Malik and others.
Anti-prediction
Website defects can not only be blocked 【010]
Diffusion channels, but also reduce the energy barrier between each other
Jump Li channel, effectively reduce Li jump
Diffusion heterogeneity
The actual synthetic olive cathode usually contains non-
Can ignore the opposite
The cause of website hair is not
Equilibrium synthesis conditions.
Although these pioneering studies have revealed significant impact defects in Li transport in LFP
Crystal LFP examined in reference.
Grown by floating zone method, unlike most LFP particles used in batteries, which are mainly prepared by hot water synthesis or solidsstate reaction.
Measurement of Li
Diffusion heterogeneity in LFP micro-scale
Particles and nanoparticles made with these methods are rare;
It is not clear the 2d Li transport behavior of single crystals reported in the literature.
Represents all forms of LFP, regardless of their synthesis method.
In order to clarify the phase transition and ion diffusion properties in electrode materials such as LFP, direct observation (de)
The process of lithium in a single particle is very valuable.
Previous studies using in situ transmission electron microscopy (TEM)
In-situ or in-situ soft X-
X-ray scanning transmission X-rayRay microscope (STXM)
, In-place/hard X-
Combined X-ray TXM
Near-ray absorption
Edge structure spectrum (TXM-XANES), and soft X-
Ray ptychographic microscope provides valuable insights in phase
Boundary orientation and movement, non-uniformity of reaction and (de)lithiation-
Induced mechanical strain in LFP.
Especially TXM-
XANES is a unique technology in large fields. of-view (
Up to 40 × 40 μm)
And the spatial resolution drops to Nano (~25u2009nm)
Can be used to study composite electrodes (
Active material/carbon/adhesive)
Use a simple battery.
In this work, we use
XANES imaging tracks the composite electrode carbon black, adhesive, and single electrode composed of the chemical process of delithiation
Specially synthesized crystal LFP Micron rod with long
Along the axis of [growth]010]direction.
Through theoretical analysis and stage interpretation
Field Simulation, our experiment provides-
Diffusion heterogeneity and phase
Boundary migration mechanisms for LFP and other regions.
First, we confirm that the tiny LFP particles synthesized by water-heat may also have similar Li-
Diffusion constant along [010]and non-[010]
Direction, support the defect-induced (transversely)
The diffusion coefficient of isogay Li is a common feature of LFP and has nothing to do with the comprehensive method.
A direct consequence of such behavior is that, based on common belief,100)or (001)
In fact, the surface of the LFP particle should be considered as an active substance in Li (de)
Plug-in process.
Second, we were able to obtain the first direct proof of the previously predicted SRL conversion behavior.
In addition, our study reveals a more subtle and complete picture of phase change in LFP and plug-in compounds, because we show that in (de)
Lithium may continue in mixed mode, in mixed mode, phase-
The boundary movement follows the SRL and BDL dynamics in different directions, respectively.