Introduction to Li-ion batteries - Part 1

I.Introduction
 - Two type are well-known for their two-phase structure: olivine LiFePO₄ + spinel Li₄Ti₅O₁₂ 
 - In comparison to olivine LiFePO₄, spinel Li₄Ti₅O₁₂ possesses a ‘zero strain’ property and performs Li-site switching during the phase transition, which lead to a different phase structure

For the two-phase reactions, the two-phase interface will limit the diffusion of Li-ions, so the electrochemical kinetics will be dependent on the phasetransition mechanism. Although this mechanism is inherent to the electrode materials, it can be substantially influenced by nanosizing the crystallites, due to the reduced miscibility gap and the increased specific surface

 - Olivine LiFePO₄: most promising cathode materials [low toxicity, low cost and high safety]
 - Spinel Li₄Ti₅O₁₂: anode materials [excellent cycle ability, high rate capability, and better safety compared with carbon anodes]
II.Olivine LiFePO₄

1.Phase diagram 

Li_xFePO₄ could be described as a mixture of the Fe3+/Fe2+ mixed-valent intermediate Li_αFePO₄ and Li_1-βFePO₄ phases, where α = 0.05 and 1-β = 0.89 for particles with a mean size of 100 nm

(a) Open-circuit voltage versus x in LixFePO4 at 258C, where vertical lines are the monophase/biphase boundary estimated from Rietveld refinement results of neutron-diffraction data. (b) Phase distribution diagrams of LixFePO4 (0 < x < 1) established from temperature-controlled XRD data

 - the formation of a solid solution is almost entirely driven by electronic rather than ionic configurational entropy in the LixFePO4, where the electronic entropy arises as electrons or holes are localized

 - However, the electron delocalization would occur at an elevated temperature in the solid solution of LixFePO4, due to rapid hopping of the small polarons

2.Two-phase structure

(a) HRTEM image of the disordered region between two phases in a thin Li0.5FePO4crystal, with Fourier transforms of the indicated areas. (b) STEM high-angle annular dark field (HAADF) image of the chemically delithiated sample Li0.45FePO4with the analysis line and the 3D representation of EELS spectra recorded along that line. (c) Full field TXM image of a selected crystal in a sample with nominal composition Li0.74FePO4collected at 7080 eV, and the corresponding chemical phase map obtained by linear combination (LC) fitting of XANES data at each pixel. 

 - a disordered region between two crystalline domains is induced by the stress fields accompanying the lattice parameter mismatch

Transmission X-ray  microscopy  (TXM)  coupled  with  X-ray  absorption  near  edge spectroscopy  (XANES)  to  produce  the  chemical  composition maps  for  a  partially  delithiated  sample  of  Li0.74FePO4

• X-ray photoelectron spectroscopy (XPS): investigated the redox processes of nanosized carbon-coated LiFePO4

 - A continuous evolution of the Fe3+/Fe2+ ratio at the surface of the particles excellently agrees with the Li content of the entire electrode upon charge and discharge.

• soft  X-ray  absorption  spectroscopy  (XAS): studied  the  phase  transformation  and  (de)lithiation  effect on  the  electronic  structure  of  LixFePO4

3.Phase transition models

the  phase-transition  models  are  usually  established  on  the  basis  of  the  strong  anisotropy  of  Li-ion  diffusion

• maximum  entropy  method  (MEM): visualized  the one-dimensional  curved  Li-ion  diffusion  path  along  the  bPnmaaxis  of LixFePO4

 - the LiFePO4 nanoplates  with  crystal  orientation  along  the  ac  facet  show a  better  rate  performance  than  that  along  the  bc  facet  

 - the  size  effect  in  defective  LiFePO4 as  the  defects would  block  the  Li  migration  in  the  one-dimensional  channel

(a)  3D  distribution  of  Li  nuclear  as  blue  contours  calculated  by  the  MEM  using  NPD  data  measured  for  Li0.6FePO4at  620  K,  where  the  brown  octahedral represent  FeO6 and  the  purple  tetrahedral  represent  PO4 units.  (b)  Schematic  view of  the  ‘domino-cascade’  mechanism  for  the  Li  deintercalation/intercalation  mechanism  in  a  LiFePO4crystallite. (c)  Schematic  of  the  shrinking-core  model  to  describe  the  juxtaposition  of  phases  in  the  LiFePO4electrode.(d)  The  hybrid  phase-transition  model  as  a  combination  of  the  domino-cascade  model  and  the core-shell  model  to  describe  the  micro-particle  of  olivine  LixFePO4

•   For  the  LixFePO4 nanoparticles: the  phase  transition  is  considerably  faster  than  the nucleation  process => proposed  the ‘dominocascade’  mechanism  for  the  Li deintercalation/intercalation  in  a LixFePO4 crystallite [the  interfacial  zone  moves  rapidly,  like  a  wave,  through  the  crystallite  in  the  a direction]. The  two-phase  configuration  would  mainly  be  determined  by  the  poor  kinetics  of  Li-ion  diffusion  in  the  Li2O  solid electrolyte,  instead  of  the  strong  anisotropy  of  olivine  LiFePO4
• For  the  big  or  the  secondary  particles: the  two-phase  configuration  is  not  only  determined  by  the  anisotropy  of  olivine  LiFePO4, but  also  the  kinetics  inside  the  particles

‘core-shell’  model :  explain  the  asymmetric  behavior  and the  path-dependence  during  the  electrochemical  processes  of LixFePO4 electrodes
 - the  nucleation  and  growth of  a  new  phase  governed  the  electrode  kinetics  under  a  small
potential  step  applied  to  large  particles
4.Many-particle electrodes

(a)  Two  possible  scenarios  of  new  phase  formation  in  a  many-particle  system. (b)  &  (c)  Hysteretic  behavior  generated  by  an  electrode  with  10  and  1000  storage  particles.  (d)  PED phase  map  acquired  within  the  partially  charged  sample  as  Li0.25FePO4 and  (e)  the  corresponding  orientation  map,  where  color  coding  is  given  below  each image.  (f)  State-of-charge  mapping  obtained  via  scanning  transmission  X-ray microscopy  for  LixFePO4electrode  powder  dispersed  by  sonication.  Outlined  in  white  are  particles  in  which  two  phases  coexist  within  the  same  particle.

 - the  individual  particles  were  charged  oneby-one  rather  than  all  of  the  particles  simultaneously

 - a  saw-like chemical  potential  profile  for  few  particles  and  the  smoothed potential  for  many  particles as Firgure b [this  phenomenon  is  due  to  the  non-monotone  relation between  the  chemical  potential  and  the  Li  mole  fraction  of  a single  particle]
 - precession  electron  diffraction  (PED) : obtained LiFePO4and  FePO4 phase  mapping  at  the  nanometer-scale  on  a large  number  of  particles  of  sizes  between  50  and  300  nm  in  a
partially  charged  cathode as Firgure d & e

the  rate  of  initiating  the  phase  transformation  in  a particle  is  much  slower  than  the  rate  of  completing  the  phase transformation 

5.Strain accomodation

(a)  Free  energy  including  coherency  strain  energy  for  two-phase coexistence  for  the  various  morphologies  as  homogeneous,  incoherent  twophase  mixture,  the  periodic  two-phase  morphology  I  and  II. (b)  Simulated Li0.5FePO4 microstructures:  Phase  boundaries  align  along  (1  0  1)  planes  to minimize  elastic  coherency  strain  (left)  and  loss  of  coherency  in  the  (0  0  1) direction  causes  the  stripes  to  form  along  (1  0  0)  planes  (right).  (c) Diffraction  patterns  of  Li0.5FePO4(black)  and  Li0.5Fe0.85V0.1PO4(red),  insert shows  magnified  (2  0  0)  diffraction  peaks  of  triphylite  and  heterosite phases